I mean I guess you could say they have different semantics. They're just different types, int and Int64 aren't any more different from each other than Int64 and Int32. You can treat all of them exactly the same, just like how you have ints and longs and shorts in C and they all have the same interface.
Regardless, I don't think C's "probably 32 bit" non-guarantee is the make or break feature that makes it a systems language. If I care about the exact size of an integer in C I'm not going to use an int- I'm going to use explicit types from stdint. Rust makes that mandatory, and it's probably the right call. OCaml isn't really what I'd use for a systems language, but that's because it has no control over memory layout and is garbage collected. The fact that it offers a 63-bit integer doesn't really come into it.
Regardless, I don't think C's "probably 32 bit" non-guarantee is the make or break feature that makes it a systems language. If I care about the exact size of an integer in C I'm not going to use an int- I'm going to use explicit types from stdint. Rust makes that mandatory, and it's probably the right call. OCaml isn't really what I'd use for a systems language, but that's because it has no control over memory layout and is garbage collected. The fact that it offers a 63-bit integer doesn't really come into it.