The question as asked in the title depends on what you consider "strong evidence". I will therefore have a look at which side makes the better-sourced point. (Which is not just about "protecting people with damaged airways", but several other health effects as well.)
One side -- the Umweltbundesamt -- gave references to studies the decision was based upon, and also discusses openly the uncertainties and compromises involved, and dissenting voices.
The UBA freely admits that there is little data on the effects of NO2 in isolation from other traffic-related pollutants. Since we don't allow human experimentation, such data would be hard to come by. But the UBA / EU limits are based in statistical data, which does show correlation, and considers causation at the very least "probable" (EPA).
The other side -- Prof. Dr. em. Köhler -- gave his personal opinion. Nothing more.
The Umweltbundesamt (German Environment Agency) article linked in the question has an extensive list of sources underlining their summaries. The most interesting sources, and the summaries made by the UBA (translations mine, parts where I am not sure about the nomenclature marked with (?)):
- WHO – World Health Organization (2013a): Review of evidence on health aspects of air pollution – REVIHAAP Project. Technical Report. Copenhagen, Denmark.
...since 2004 numerous epidemiological studies published that show correlations between NO2 short-time exposure and mortality, hospital visits, and respiratory symptoms. The results of these studies were reproducible in chamber experiments (?) and hint that a new recommendation for the short-term NO2 limit might have to be set lower than the current limit. The WHO brain trust recommended an intense review of scientific works on long term exposure, but pointed out that the effects of NO2 could so far not be completely separated from those of other traffic-related air pollutants.
Emphasis in the summary is mine:
WHO committees consulted current European population surveys studying correlations between adult mortality and yearly average NO2 concentrations of below 40 µg/m3. They concluded that health impacts of NO2 have to be accounted for starting from a long-term average exposure of 20 µg/m3.
The effects of long-term exposition to these low NO2 concentrations is discussed controversially among scientists. In 2016, the US Environment Agency (EPA) published a systematic overview of NO2 studies up to 2014, with special focus of separating the effects of NO2 from other pollutants. The correlation between short-term NO2 exposure and asthma attacks is considered to be causal. The correlation between long-term NO2 exposure and the onset (?) of asthma is considered "possibly" causal. In the eyes of the US EPA, there are not enough significant studies showing an effect independent of other pollutants. The same is true, according to EPA, for the studied effects of cardiovascular diseases, diabetes, reduced pre-natal growth, cancer, and mortality.
The UBA then adds its own conclusion:
Independent from this discussion about the significance of the long-term exposure to NO2, which is somewhat controversial in Germany too, it remains certain that traffic related pollutants have detrimental health effects, regardless whether these are caused by NO2 in isolation or in combination with other pollutants. NO2 is and remains a meaningful indicator for traffic induced pollution. For this reason alone, the limit for yearly NO2 exposure serves its purpose for public health care.
Prof. Dr. emeritus Dieter Köhler
This gentleman has gone on record with his views about pollution and the invalidity of current exposure limits on multiple occasions and in various media. (Though he never published scientifically on the subject.)
Article, attribution unclear:
The 40 microgram recommendation of the EPA referred to indoor values and the assumption that susceptible persons like infants would breathe this air all year long. The WHO took the 40 microgram recommendation and without further ado made it the limit for outdoors.
This makes it sound as if outdoor limits should not be as low as indoor limits... which is evidently nonsense. For comparison, the limit for workplace exposure in Germany is 950 µg/m³. Indoor conditions can lead to much higher NO2 concentrations, while it is pretty hard to picture how inside concentrations could be any lower than the ones outside. This statement in the second paragraph makes me look askance at the whole article from the get-go.
Köhler is quoted:
"They just made a city-country comparison, found minimal differences in life expectancy. There are of course many, many other reasons why people in the country live a bit, a couple hours longer."
I leave the evaluation of that statement to the reader.
"NO2 is especially harmless, because it is formed in the body itself, from NO. The organism is used to dealing with it. It's reducing it."
Our bodies form CO2 as well, that doesn't mean we can tolerate arbitrary concentrations of it in the surrounding atmosphere. The same is true for many other things our bodies (attempt to) get rid of because they are not healthy. Our bodies expend energy trying to get the concentrations lower. That should be a hint.
The quote continues:
"And it is not traceable why the little bit of NO2 you inhale should result in diabetes."
Please excuse a bit of personal opinion at this point. This is like listening to one of Mr. Trump's speeches. "I don't understand it, I don't like what it would mean to me, so it must be untrue."
All we have to go by are studies. These studies are correlating NO2 exposure with detrimental health effects. Admittedly these studies also correlate a number of other variables with these health effects, because we don't allow human experimentation and NO2 is hard to isolate from those other variables.
But just because we don't know how the mechanism from pollutant to health effect works exactly does not mean it doesn't happen.
And what qualifies as "a little bit" (i.e. negligible even in the long run) and what is a significant amount is the very issue at hand.
Syren Baran stated in a comment that he downvoted this answer for not addressing "the claim about smokers". I dismissed that claim because it isn't part of the question, I could not find hard numbers for the claim in the first place, and because I did not want to engage in original research to debunk it.
The German newspaper taz published an article about the various mistakes in Dr. Köhler's claim.
Core points from that article:
Köhler claims (in writing) 500 μg NO2 per cigarette, inhalation of 10 liters per cigarette, therefore 50,000 μg NO2 per cubic meter, giving 1 million μg NO2 inhaled per pack of cigarettes.
There's a calculation error in there. 500 μg / cigarette times 20 cigarettes per pack gives, by necessity, 100,000 μg / pack, not 1 million. (Exaggerating the NO2 inhaled by smoking by a factor of 100.)
There's an error in the premise, as 500 μg is not the amount of NO2 (nitrous dioxide) in cigarette smoke, but the amount of NOx (all nitrous oxides).
Dr. Köhler initially claimed that 10% of the nitrous oxides in cigarette smoke were NO2, making his calculations off by a factor of 1000.
In a later mail to the editor, he claimed a range of 10-50%, making his calculations off by a factor of 200-1000.
So Dr. Köhler's claim -- that the NO2 limits are nonsense because (paraphrasing) in 80 years of inhaling NO2 at those limits, you'd be exposed to as much NO2 as in less than two months of smoking -- is wrong.
Those 80 year's worth of 40 μg NO2 would be (depending on the fraction of NO2 in a cigarette's total NOx) more like 6.4 to 32 years of smoking.
The article goes on to point out similar errors in Köhler's calculations on particulate matter in cigarette smoke, and ends with quoting his excuse:
Ich mache ja praktisch alles allein und habe nicht einmal mehr eine Sekretärin als Rentner.
My translation: "I am practically doing all of this on my own, as a pensioner I don't even have a secretary."