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.
And I'm not sure I understand why you're using the inability to describe certain actions as the reason why you use ISO C.