Anthropic has done plenty of cheap marketing tricks as of late, see their recent non-functional C compiler that relied on a harness using gcc's entire test suite
It is functional. You can try it yourself or find third-party tests of it, even. Why do you think that it's a "cheap marketing trick" to test it on the GCC test suites?
Not surprising given that they dont even know why claude-code works as before or doesnt work [1] ie, there is no known theory of operation. Explains why they are afraid of it.