I'm not even sure it does work. Prime only tried trivial packages, but based on my experience with agentic coding, I'm not convinced they're able to deliver a fully functional package of medium size while being truly clean room.
Also, there's an anecdotal comment on youtube of someone who tried this (on a small package) and they mention the clean room was violated (the AI added implementation instructions to the documentation), had performance issues, and the best part, the generated code had an MIT license. Now I wish Prime had looked at the LICENSE files created.
afaict they just run the LSP automatically when the agent uses the file edit tool, passing errors/warnings as a response of the tool. Maybe they run it before and after to get only the warnings introduced after the change, maybe they filter by the lines changed, I'm not sure.