One's not a blocker to a malloc implementation since you could otherwise just cut up a static char buffer. Coordinating with the OS is a nice to have. Strict aliasing is hard blocker. Hence why that's the issue under discussion.
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.
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.
That is why I tend to always write ISO C instead of C.