Implementations of C have more functionality than C itself - like inline assembly or syscalls or machine-specific intrinsics, so it can do more. ISO C only has what's written in the standard.

(A syscall is an example of "something you can only do because the implementation isn't visible to the caller" - it can violate aliasing that way.)

Also, my argument isn't about type aliasing, it's about UB on out of bounds pointers. Could be some other aliasing issues though.