Completely conclusive proof? No, but it seems unlikely that there ever would be conclusive proof of such.

They don't seem to exist outside of this incident and things related to this.

There were multiple people who also don't seem to exist outside of their posts to the xz mailing list applying pressure for the original maintainer to bring Jia on board. This occurred around the time that Jia was first making contact with the project, not only recently.

Apparently the IP addresses that were logged for Jia is a VPN based in Singapore

They have vanished.

Honestly, there's very little evidence that they weren't always intending this.