As Garstecki notes, most headphones are capable of outputting up to 125dB1 of which prolonged exposure can result in hearing loss.
To get an idea, look at this sound comparison chart:
Sound level comparison:
City Traffic (inside car) 85dB
Train whistle at 500' 90dB
Truck Traffic 90dB
Subway train at 200' 95dB
hearing loss from sustained exposure 90 - 95dB
Power mower at 3' 107dB
Snowmobile, Motorcycle 100dB
Power saw at 3' 110dB
Sandblasting, Loud Rock Concert 115dB
Garstecki's claim is based of studies on this very topic, many of which he was directly involved in.
From Comparing two methods to measure preferred listening levels of personal listening devices
Preferred listening levels for self-selected music were determined in
quiet and background noise using a probe microphone, as well as in the
DB-100 ear simulator mounted in KEMAR. The ear-canal measurements were
compensated for diffuse-field. Only one of the subjects was found to
be listening at hazardous levels once their reported daily usage was
accounted for using industrial workplace standards.
From The Effects of Listening Environment and Earphone Style on Preferred Listening Levels of Normal Hearing Adults Using an MP3 Player:
The majority of MP3 players are sold with the earbud style of
headphones. Preferred listening levels are higher with this style of
headphone compared to the over-the-ear style. Moreover, as the noise
level in the environment increases, earbud users are even more
susceptible to background noise and consequently increase the level of
the music to overcome this. The result is an increased sound pressure
level at the eardrum. However, the levels chosen by our subjects
suggest that MP3 listening levels may not be as significant a concern
as has been reported recently in the mainstream media.
As Garstecki also notes in the passage quoted in your question, some people are more susceptible than others. The risk varies per individual.
A 1993 study acknowledges the wide variance in susceptibility to hearing loss. From Individual susceptibility to noise-induced hearing loss: an old topic revisited:
The wide range in susceptibility to noise-induced hearing loss has
intrigued researchers and hearing conservationists alike. Some of
these differences in variability have been attributed to various
intrinsic factors such as eye color, gender, age, etc. However, a
review of controlled research shows that the influence of these
intrinsic variables is relatively small and cannot explain the wide
range of hearing loss observed in demographic studies.
The studies above found that most people don't listen to music through headphones at a hazardous volume. There is little doubt that it is possible to listen to music through headphones at a hazardous volume, which could damage hearing.
1 - Not the most reliable reference, although it supports what I have found when looking up the average output of headphones.