Copyright doesn't protect ideas, it protects writing. Avoiding reading LLVM or GCC is to protect you from other kinds of IP issues, but it's not a copyright issue. The same people contribute to both projects despite their different licenses.
They don't call Clang a "clean-room implementation". Unlike Anthropic, who are calling their project exactly that
A clean-room implementation is when you implement a replacement by only looking at the behavior and documentation (possibly written by another person on your team who is not allowed to write code, only documentation).
