"He is not my friend, he is an acquaintance."
Translation:On nie jest moim przyjacielem, on jest moim znajomym.
I think it is just because "Kolega" is more common than the English equivalent. I don't think it is used as friend but sometimes Americans, for some reason, don't like to call someone from class or work a colleague. Usually we say a coworker or a classmate. Sometimes people say friend very loosely, true…
I don't know, maybe it all depends on where you were raised and all. I've never really heard my parents talk about someone as a kolega if they don't consider them a friend. Could also be that I don't know Polish as well as a Polish Ameriacn should and that's why I'm on Duolingo xD
You can interpret "dorosły" both as an adjective (in your example) and a noun. Although "On jest dorosłym" (treating it as a noun) does sound strange. I guess it's mostly an adjective.
"Znajomy" also can be interpreted both these ways, but it's mostly a noun. You could say "znajomy mężczyzna", but mostly you will just say "znajomy" (to mój znajomy, twoi znajomi, znajoma mojej siostry), treating it as a noun. "On jest znajomy" on its own sounds even stranger than "On jest dorosłym".
EDIT: Well, I guess that as an adjective you'd translate "znajomy" to "familiar", and as a noun to "an acquaintance".