True, but it has no bearing on what this person was taught when they learned English. It's entirely plausible that some English teachers, influenced by the criticism that arose in the 19th Century, teach that singular they is to be avoided.
Sure, I'm not attempting to say that this person was not taught that singular they was not a proper usage, just that it has been a proper usage for longer than any of us have lived.
I learned English in the context of a native language with lots more structure about genders, forms, cases and tenses. So I guess that it was something my teachers missed. And I went from that to immersion in American culture.
https://en.wikipedia.org/wiki/Singular_they