It's entirely possible that executing a Python program could cause C code in the Python interpreter, or C code called via FFI, to do something that the C spec defines as undefined behavior. That could affect the result of the Python program. If that happened, it would not be reasonable to say that the Python program did not have undefined behavior -- even if nothing in the Python code was undefined according to the Python spec.
It's entirely possible that executing a Python program could cause C code in the Python interpreter, or C code called via FFI, to do something that the C spec defines as undefined behavior. That could affect the result of the Python program. If that happened, it would not be reasonable to say that the Python program did not have undefined behavior -- even if nothing in the Python code was undefined according to the Python spec.