There are essentially two definitions of racism at this point.

The colloquially version, which means "prejudice based on race" and a second version, which specific groups and people have advocated for, which means something like "structural oppression through cultural and governmental means". It's more complicated than just that, but it's a fairly narrow term for them.

So when one person says "there's no such thing as anti-white racism", you hear, "No one's prejudiced against white people for being white!" Obviously, that's ludicrous.

But that person is likely using the, I have no idea what to call it, "advocate definition" maybe, definition would which preclude anti-white racism from existing within that narrow definition of racism.

So it's a debate where people aren't speaking the same version of a language, convinced each other are uninformed, reactionary or stupid.