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.

> Constitutionally, only the federal government is allowed to regulate intellectual property

It turns out that's not exactly the case! See, e.g., Goldstein v. California, 412 U.S. 546 (1973). Before 1978, state (often common law) copyright used to cover a lot of pre-publication works, and until 2018 (when the federal law was amended) state copyright law covered pre-1972 sound recordings, and state copyright still covers obscure things like post-mortem moral rights in visual art or rights to "unfixed" works. See 1 Nimmer on Copyright §§ A.02 & 2.02. Other forms of intellectual property (trade secrets, rights of publicity) remain mostly creatures of state law, and some states also have trademark systems.