Yeah, it's pretty cut and dry. Constitutionally, only the federal government is allowed to regulate intellectual property, so re-implementing anything that isn't protected by a trademark, copyright, or patent is fair game, and trademarks don't cover design, copyright only covers media, and patents expire in 20 tears.

Even the clean-room isolation that Phoenix went through isn't legally required, it just makes nuisance lawsuits more difficult. BSD prevailed over UNIX System Laboratories, in their reimplementation of Unix, despite having directly worked with the source code.