Admittedly, I haven't been playing or lurking since Thanksgiving so it's within the realm of possibility that something libelous per se regarding pro was posted that I didn't see. I highly doubt it, though, as mere opinion would not rise to a proper level. Even if something like that was posted, it would have had to be seen by somebody who held him in moderately high regard beforehand to have damaged his reputation. So, he would have to find somebody in the CF community that would say that he had a reputation capable of being damaged. (Hahahahahaha.) Or he would have to prove that somebody who doesn't read these boards somehow saw the libelous information. Of course, if Pro self-published this information to the 3rd party, there is no actionable claim. Many types of libel claims also require a plaintiff to prove monetary damages, which seems unlikely in this case.
So, if Pro can prove somebody who does not use these boards somehow saw the libelous information without Pro showing them himself, and that information directly caused him monetary damages, he may have a case against the original poster of said information. That doesn't seem likely, but neither did Pro being a cabal leader, so who knows?
tl;dr - Pro's most likely somewhat erroneous in his logical construction, like always.
Until you get a letter from a lawyer representing him, or proper notice that he's filed a claim of some sort, tell him to provide you with statutory authority for what he's threatening, or to kiss your hairy ass. Feel free to PM me if you have specific questions.