Those implementation details are what define the differences between
languages, therefore the claim stands. If you did the legwork to
extend INTERCAL to be able to call C libraries, the resulting language
would no longer be INTERCAL, but INTERCAL with extensions;
alternatively, if you wrapped INTERCAL with a shell that parsed the
output of an INTERCAL algorithm and invoked an outside function based
on that, you haven't made INTERCAL able to do it but instead you've
constructed a system that can do it using another language.
