The claim was: The same goes for C++ or Swift. Getting the function pointer for a method can be involved, but all functions adhere to the C ABI.

This is clearly false since at least one C++ compiler generates functions that does not adhere to any of the common calling conventions for C on that platform (or "C ABIs" if you will).

Besides that, I said you could not call them from C. An asm stub is not C. Neither is inline assembly even though it's a common extension to C.