> it’s completely bizarre to assume

I assume this all the time on Reddit. It's not bizarre at all.

It's not right, sure. But it's not bizarre.

> It’s literally no skin off your nose to write “they”

"They" is traditionally a plural pronoun, so, either we knowingly make a grammatical mistake and introduce possible confusion there or we just take a 50/50. My English prof, a very snooty old white dude with a lot at stake in such trivial grammatical affairs, suggested using "he" half the time and "she" the other half - on different essays/posts of course so as not to be too confusing.

Whatever side you come down on this - and there is no objective right or wrong here - the one thing we can all agree on is that it's trivial and only pedantic cretins like me (and apparently you) worry about it.