I don’t disagree that he has been a correct singular pronoun for unspecified individuals going back centuries, just that they wasn’t. They has been used just as legitimately as a singular pronoun going back as far as it’s been in common use. I think especially in such places as it’s convenient to the writer to assume the one being referred to is male, he took prevalence. However, I would strongly disagree that he was objectively correct over they in historical contexts, rather that either could be used depending on the author, their instruction, biases, and intentions. Otherwise I agree, and they is certainly better than “he or she.”