Even if Apple had basd their watchOS toolchain on GCC, and had published the source code for it, it is quite likely that their fork would have remained separate

Even if Apple had basd their watchOS toolchain on GCC, and had published the source code for it, it is quite likely that their fork would have remained separate to upstream: upstream might not care about watchOS support, and Apple's fork might be based on an old version of GCC. In that case, Rust would still have to support two versions.