system
A chat on subreddit /r/NoStupidQuestions
human
Especially since they don't use the name socially.. it would seem like whatever they're hiding isn't just social media/a job/university etc. but something involving their legal name - my mind jumping to criminal record.

Not saying they are wrong or weird, but I don't think the other person is wrong for finding it weird either.
gpt
Either a public sex worker that used their real name, someone with a substantial criminal record, or is a registered sex offender.

What else would people care about hiding?