A Comm for Historymemes

You touched on the point there, although you might argue it's only a semantic difference. If you accept that "white" isn't a race, then definitionally, you cannot be racist against white people. Just like you can't be racist against Muslims, at least on the basis of their religion alone. You can definitely discriminate against those groups, but calling it racism is simply incorrect.

You can also delve into a discussion about the harm done by this kind of joke. The classic metaphor is that you don't blame the bullied kid for making fun of their bullies. There are power imbalances built into our culture, and so pretending that a joke at the expense of white people could ever do the same amount of harm as one at the expense of a racialized group is frankly ridiculous.