You're asking a lot of questions and they aren't as specific as they probably should be for a site like this. Let me just cut to the chase, and then try to justify my answers.
- Can anyone reasonably expect to obtain a physique like Arnold without drugs? No
- Can anyone reasonably expect to obtain a physique like Arnold with drugs? No
- Can tall, skinny guys get more muscular and stronger? Yes
- Are ads on TV and the internet misleading or out-right lies? Yes
Those are my answers, at a glance. Now comes the justification.
First, we need to make it clear that we are talking about 'average' people (as per the question) not outliers. An average person who takes up sprinting can train and improve - but WILL NOT EVER run as fast as Usain Bolt. It's safe to say that because, empirically we see that the world's best runners cannot run as fast as Usain Bolt.
Second, I'm really not sure this question should be on this site. It's easy to say X did Y but it's very hard to say it is physically impossible for X to do Y. Can some ficticious guy run a four minute mile? Well, there was a time when nobody had - and some people argued it was impossible for medical reasons, while other people argued it was achievable. Then some guy did it; but he was the only one. Now world class athletes can do it. But what does that mean for the hypothetical question of 'Can some guy run a four minute mile'? So, I'm doing my best to answer this question with actual data - but it does get pretty speculative. Hopefully someone else can do a better job.
Let's have a look at Arnold
I think it's really important to clarify what we're talking about when you say 'Like Arnold'. Let's try to get some perspective.
Arnold is one of the world's most successful body builders. He won The Mr. Universe three years in a row and the Mr. Olympia six years in a row. He was also a world-class, competitive power-lifter, taking first in the International Powerlifting championships and the German Powerlifting Championships.
He was competing against the other best body builders in the world and he consistently beat them. That means, all of the professional body builders competing in the Mr. Olympia, for six consecutive years, were unable to obtain the physique as good as Arnold's.
But let's talk about drugs. The question specifically asks about what we can achieve without drugs....
The truth is, Arnold took steriods. He admits it. The people he was competing against also took steroids. Many of them weren't illegal or banned at the time. '...the use of anabolic steroids was openly discussed partly due to the fact they were legal...'
In Arnold's own words, 'It was what I had to do to compete'. This would strongly suggest that Arnold probably could not have achieved his physique without drugs.
Bottom line is, EVEN WITH STEROIDS, none of the world-class, professional body-builders could obtain a physique like Arnold's. Arnold still dominated the sport. Yes he used drugs too. We can't say with absolute certainty that Arnold without drugs couldn't obtain his physique - but we can say he didn't do it and that he believes he couldn't have. Arnold himself, who without drugs was a world-class powerlifter, most likely couldn't obtain his physique without drugs.
You'd need to be an extreme outlier and take drugs to get the physique that Arnold is famous for.
What About Our Tall/Skinny Guy
Okay fine - so we're not going to look like Arnold, with or without drugs. But what can a tall/skinny guy hope to achieve by working out? Well, there are lots of health benefits...
Controls weight, combats a whole slew of negative health conditions/diseases (arthritis, diabetes, osteoprosis, obesity, back pain, depression), increases muscle mass, improves sex life, and promotes better sleep.
So I mean that's pretty good right? Reason enough for any tall/skinny guy to head to the gym!
But I feel like that's not what you're looking for. Still, I'm having a lot of trouble finding sources that seem appropriate for this site. You can find a lot of different formulas online that will claim to tell you how much lean mass you can 'naturally' support. You'll notice that they all give slightly different results and most of them don't explain or justify the methodologies used in their creation.