I am wondering how that applies to newly generated code.

Odd variable naming? Stylistic choices that are watermarked?

Or as someone else noted further down in the comments, it could be more subtle:

Between the first and second most likely choice, in certain positions it will consistently choose in a certain way.

I cannot imagine the code with well defined specification will have extra watermarks unless the watermark is requested as part of the harness instructions.

If it works like people describe - on the every nth token or something - then the mark will be left in the chain of thought and discussion with the model - not in the code artifacts.

I would guess they're not worrying about watermarking a tweak to a human-written program. That's both a tiny fraction of Claude use and of very little concern to the kinds of people who want to check watermarks.