I think it's just a coincidence.

- All the posts are from 2004/2006. - "jiat" can be abbreviation for many common Chinese names.

I agree, probably a coincidence. Just wanted to point out we can, actually, find the username online.

It might just be a coincidence, but the same username from that gmail account also appears to have a Proton Mail address

I think it's not a coincidence: Hans Jansen (hansjansen162@outlook.com) has a matching account on Proton mail too (hansjansen162@proton.me). Furthermore, the Outlook account is configured as recovery e-mail for the Proton account.