[EDITED to add:] As pointed out by a helpful comment below, I was misremembering: Anthropic do apply their watermarking to code, they just say that it will have negligible impact on the actual code (because there's generally less scope for variation in that) but e.g. it will have its usual effect on comments in the code.
> code—which in very many cases has to be exact—has generally less watermarking than some other forms of text.
Generally less watermarking. Not no watermarking.
It’s just much less effective since code is low entropy.