Prototypal inheritance is OOP. It's just that we tend to think that the only type of OOP is class based inheritance.
JS didn't change, it still uses protypal inheritance and as you mentioned, they added classes mainly as syntactic sugar.
You have objects, they can encapsulate state, they receive messages from the outside using their public interface at runtime, that's more or less all there is for a basic OO language.
If you look closely at Python source code you'll notice how one in theory could implement object orientation directly in C, this is the constructor of a Python object:
op = (PyObject *) PyObject_MALLOC(_PyObject_SIZE(tp));
if (op == NULL)
return PyObject_INIT(op, tp);
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