Its racist for a number of reasons. One is that Chinese is a real thing as a racial distinction in all reality even if its not technically, as something that is socially constructed, but still very much real. That is to say as a practical matter there are clearly Chinese people, and we can clearly be racist to them on that basis, as a result of their culture or language or stereotypical things to do with appearance, and such. Also just as a matter of common usage and colloquialism racist can clearly be referring to nationality, even if its incorrect technically at one level it is correct at every other level.

I mean the hypothetical black person you mention. Sure, technically a racist can no longer attack his race. But every other part of his Chinese culture, language, customs, mores, everything else can be attacked. The person was born into that, and has no real choice in it, and is being attacked for it, in something that socially is in all ways like a race. I think it would be pretty normal for the black Chinese guy in your hypothetical example to feel the person is pretty darn racist if they were attacking every possible stereotype of Chinese people. Ideally of course your right, race is not a real thing, and gender is largely not a real thing, certainly as we define it now.

But as a social construction it very much is a thing, its a thing because racists, and centuries of racist imperialism we only stopped overtly a few decades ago, made such distinctions real. Technically there are no races at all, no black or white or whatever people, because if you do a DNA test every human being has a bit of other DNA strains in them, the only exception to this is tiny tribes that have no real contact with the open world. So technically race is not real at all, in the strictest sense. But in reality obviously race is real, because racists and structures and all society sees race as real. Same as Chinese people, clearly Chinese people are a thing, and clearly you can be racist to them in all normal, reasonable definitions of the word racism, simply as a practical, common sense matter.