Aren’t organisations like Microsoft continually saying already like 30-50% of their code is now written by AI?

This means they no longer hold copyright on their code.

That's true if you assume that any LLM-written code is not copyrightable by the entity using the LLM. Everything about reality currently suggests that this is a false assumption, and AFAIK no court has made a ruling saying that LLM-generated code isn't copyrightable. Also, there are patent and trademark considerations that are entirely separate from copyright.

My point above is just that this ("ChatGPT, make me a clone of Paint Shop Pro 4 but for modern Linux and in Rust. Here's a copy of the executable to get you started.") is a much more straightforward, old-fashioned kind of copyright infringement. I don't see why a court would treat it as different from "Let me decompile paintshoppro.exe to an IR and then recompile the IR for a Linux."