It's a junk article, likely written by AI in part or entirely. Paragraphs and paragraphs of nothing just to reference a support article they found, all the while subtly implying a Windows account is a really good thing to use and everyone should use it.

You’re not wrong or anything, but “on accident” is used commonly in American English, so the author isn’t wrong either. I think it might have come from an association with “on purpose”, as in “I didn’t do it on purpose, I did it on accident.”

Unfortunately (or fortunately, depending on how you look at it), the rules of language only matter if people actually stick to them. Language shifts over time no matter who kicks or screams about it.