It probably means you're using your real name and/or your real phone number. Or have an established account with photos of you.

I did not check if images from thispersondoesnotexist.com would work to pass the selfie check, but my guess is that they wouldn't. A professional portrait of someone from a Western country who has never been seen on any photos Facebook owns, not even in the background? That's a red flag. Also, my guess is that Facebook knows about that page and takes precautions.

Anyway, if Facebook tries that hard to make sure I'm not there, why should I join?

I'm using a psuedonym but my real phone number. I don't see what they would compare it to though. It might as well just be a burner phone.

I do not believe they check it; if you provide an ID scan, it's good enough.

Because you have no choice but to join.