It looks like that user won't give any additional proof (screenshots, wallet address, txid, etc.) anytime soon as long as Unijoin will not directly respond to him or on that thread. Well, since the user is open to chat in the live support, Unijoin won't create an account there just to respond "baseless" accusation. That accusation will remain accusation thread attempting to tarnish the website's reputation but it will stay just that.
Idk the reason of him donating such amount, if it's only for attention, then he really succeed for to that.