Not just photoresist, Japan is an integral supplier of many components, chemicals, precision tools that go in all sorts of stuff. Even if everything could be replaced, it will have a big impact at least temporarily on China's own supply chain.
I can bet this will be implemented selectively against Japan's MIC only, not as others are saying to deindustrialize Japan or the like.
As long as China has replacement it is worth the short term disruption to kill Japan's industry, I don't think I need to spell out the obvious part.