that's not the kicker, it's not like he did it once and I continued to harp on it. he continued making the accusation for years, at me directly and at anyone who didn't share his worldview, so I continued mocking him for it for years and I make no apologies for it. And it's not just MC - in fact, he's probably not smart enough to have come up with it on his own. Conservatives have the accusation hurled at them constantly, he probably picked it up on some leftist blog like all his other unoriginal ideas.
and no, I don't think you should have reacted the way you did. Don't be offended by such absurdly false accusation, no matter how ugly - everyone else is smart enough to know he's full of shit (except maybe Not White Vic, he's quite fond of the tactic). The accusation itself is a clear indication that he's lost the argument, don't feed into it. Reject it outright and mock him for the weakness of his argument.