Nice pedantry. How does it prevent multiple coexisting mutable references?

It’s not pedantry. Pointers and references are truly different things.

A normal C compiler actually has to worry about a lot of extreme cases.

Imagine a function takes two pointer arguments. What if they point to the same thing? What if one points to the other and is used for a write?

The nature of the code generated varies greatly if these are possibilities…

I'm not sure you need to? Rust references don't exist in C, after all.

That being said I wouldn't be surprised if there were other factors that would weigh against considering FFI calls to Fil-C safe by default (e.g., Fil-C's somewhat looser bounds on what is considered a valid pointer access).