I know exactly what this is actually because i've seen it before.
There are services out there that do paid message board posting. You pay these people and they start posting to your forums, just to sort of generate activity..
Since forums are about all sorts of things, these people cannot possibly be able to make relevant posts to every forum. So one thing they do is look for similarly themed forums and just mirror certain posts. I'm almost certain that that's whats going on here.. The guy at the other forum probably bought some paid posting.. If anything the OTHER webmaster should be sort of pissed for getting plagiarized material..