Not enough evidence here. Your identity hide on the domain details. Just one thing matching is both domains had registered from GoDaddy. But fortunately, it doesn't prove anything. Most likely you are right on your claim, but it hasn't backed with enough evidence.

I am not sure if it would be a solution to prove, but you can try it to convince OP that you are the owner of both websites. Just put a noticeable note on both sites and ask the OP to take down this thread. Only he could help you if you may provide reasonable evidence according to OP's expectation.