I'd dispute the implication that the socialization one receives in a middle school or high school environment is always positive.
Indeed. This citation is almost always anecdotal, and if we are going by anecdote, the homeschooled kids I know are much better at communicating with adults even if some of them have a somewhat harder time talking to other middle schoolers.
I've noticed the same about only children vs. kids with siblings (only child here, and to this day atypically comfortable interacting with people significantly older than me).