COM can be as simple as that implementation side, at least if your platforms vtable ABI matches COM's perfectly, but it also allows far more complicated implementations where every implemented interface queried will allocate a new distinct object, etc.
I.E. even if you know for sure that the object is implemented in c++, and your platforms' vtable ABI matches COM's perfectly, and you know exactly what interfaces the object you have implements, you cannot legally use dynamic_cast, as there is no requirement that one class inherits from both interfaces. The conceptual "COM object" could instead be implemented as one class per interface, each likely containing a pointer to some shared data class.
This is also why you need to do the ref counting with respect to each distinct interface, since while it is legal from an implementation side to just share one ref count for it all, that is in no way required.