Completely correct, that *any* GPL is banned on Apple market.
But remember, you don't need to opensource *ANYTHING* you build with GCC. This is sad, but it is fair and true.
The real reason is quite simple - GCC is built and licensed from ground up in a way to prevent closing itself, or parts of it, or its extensions down.
Apple can easily use some closed source LLVM extension to build its own parts, and you will be required to purchase it and be dependent on it.