I mean... yes, it seems reasonable to learn social skills from a school setting? Interacting with other people, some of whom dislike/disagree with each other, interacting with other adults, etc. This seems like a reasonable step toward what you describe as 'the adult world' - which, yes, is rather different from school, but that seems a good thing; throwing a child into 'the adult world' without preparation would be crazy, right?
(I'm not trying to deny that one can learn the necessary social skills while being homeschooled, just disagree with the implication that school is not also a good place to develop them)