What makes you think that's what the article says that it did? There's a lot of specific nuance and it doesn't say that anywhere. In fact it speaks of making a test suite pass only. This is the classic cleanroom bios from specs approach but no need to extract it as the test is available to run and there's nothing in the GPL that suggests that running a test suite infects software that you run it on.
You've read books and they are in your brains corpus. You only infringe copyright if you reproduce the same actual words from the books in your memory (and then do infringing acts defined by copyright laws with that output).
Here that's not happening. The code being produced by the LLM is Rust, not C.