My understanding is that "one shotting" a C compiler from a single prompt would not be covered by copyright, but a large project like GCC containing thousands of individual contributions which each require human input most likely would (even if some of the code is LLM generated).

Also, it's worth noting that the USCO does not actually have the final say here. It's possible to register works that don't hold up in court or to fail to register works that do hold up. It's the courts that ultimately decide what the law is.