Isn’t that false? I thought the fact that you could is basically the principle behind Chinese wall reverse engineering.

Emphasis on "wall" here. People who are being careful (and when your potential opponent is as big and powerful as Apple it's best to be careful) don't let the engineers who have seen the secrets talk directly to those who are doing the clean-room reimplementation. The first group documents what they found and how they found it, lawyers check the report for anything iffy, and if the implementers need to talk to the first group they don't do so directly, everything goes through legal.

But better to get legal advice on this than just listen to some random HN person (like me).

That interpretation of "clean room" is largely a myth in the context of open source reverse engineering projects. That level of rigour only really applies when you have companies directly opposed to each other, e.g. someone making unlicensed games for a game console by reverse engineering the DRM.

Clean-rooming is not a legal requirement to avoid copyright infingement; it's a legal defense against claims of copyright infringement.

In practice, the goal when doing open source projects like this, without a legal team to consult, is to use methods and approaches that yield a result similar to strict clean-room RE and result in an end product that is not a derivative work of the original code. The tricky thing here is that you need to be able to trust that the people doing the work knows how to do this safely.

That may apply where patents are concerned.

For copyright, there really is no need to be as paranoid. It doesn’t protect some generic logic, only the particular rendition of it.