You say that like “no punching down” is an unbreakable rule of comedy. Maybe in your opinion it should be but I don’t think that’s ever been true in reality, certainly not for big name comedians as a collective.
Besides, that’s only your interpretation of the situation and it requires that you assume Dave actually believes everything he says in his comedy shows which is demonstrably untrue for other subject matter he covers. You don’t assume he rapes kids even though he made a joke in that same special where that was the premise. Without that assumption there is no controversy so maybe we should stop assuming the worst about people’s intentions. That way we don’t have to concern ourselves with pointless conjecture.
Don’t get me wrong, I didn’t think much of his trans material was very funny, but that doesn’t mean I have to jump to the conclusion that he’s a piece of shit like the internet wants me to. He’s a comedian with an incendiary style which makes it quite literally his job to say stuff like that.