He absolutely assisted in his defense, if you believe his story, at the behest of Dershowitz.
And even so, any one of us can hire an expert witness to make a statement like that. Especially big-shot professors. They aren't cheap I bet, but they surely don't cost 6 million bucks per line.
Though Pinker himself has said he regrets having provided that aid, so I don't think he himself fully agrees with you if you want to argue against it being something he should not have done.