What you are saying here is that it isn't a huge problem in practice, and I agree with that. However, that's not what the original post was saying. It said that OCaml was especially suited for writing compilers because of its ints, which is the exact opposite claim.
a) all that common to need full width integers while compiling even these languages, and
b) all that problematic to use boxed numbers in this context, or big_int.