It reinforces the misconception that "du" means "some". There is no equivalent word in English for the partitive article ("du").
The filler word "some" can sometimes be used to smooth over the gap that can sometimes be left in the flow of a sentence by the lack of a partitive article, but here there is no gap to fill.
Cheveu/cheveux is hair that grows from the human scalp. Poil/poils is hair that grows anywhere else.
in english, the word 'hair' can mean one single hair, or multiple hairs. when we say 'a man has hair on his chest' we most likely mean he has multiple hairs on his chest. we could just as well say 'this man has hairs on his chest'. i imagine the same thing is going on in french -- du poil, singular, or des poils, plural, can be used to communicate the idea of 'some hair'.
It's not quite the same thing. You could say "C'est vrai, cet homme a du poil sur la poitrine, mais il n'a pas la poitrine poilue.".
Outside of that specific kind of context, I believe that it is true to say that a native speaker would be more likely to refer to "le torse velu" than "la poitrine poilue".
If you're talking about "la poitrine" vs "sa poitrine", I think it's because in French you usually don't use possessive for body parts if it's obvious who the body part belongs to. In this case, it's obvious we're talking about the man's chest, so we don't have to specify it's his. That's why the English translation would be "his chest", even though it looks like "the chest".
sur "la" poitrine: pas sur "sa" poitrine , pourquoi utilisons "his" dans se cas ?
les cheveux = tête, on ne parle plus de cheveux pour les parties intimes par exemple ou sous les aicelles et autres endroit, pour les animaux également, on ne parlent pas de cheveux mais de poils, de plumes, de pelage ou de fourrure.