I think of that like I think of the anti ain’t and anti Oxford comma stances. They weren’t entirely correct, they were enforcing the style of the time for educated use of English. Today educated use of English still doesn’t include ain’t, but it does use the singular they for people of unknown or nonbinary gender, and it uses the Oxford comma.

The language keeps evolving and stuff like this is part of that. Hell at one point the singular they was far less controversial than the singular you