I feel like at least some of this depends on whether the ICE in a hybrid is coupled directly to the wheels or not. I believe in all current consumer vehicles it is, but there are other applications like hybrid trains and city buses where the engine is just running a generator at a constant RPM. Surely that kind of configuration would be the absolute best case scenario for ICE efficiency, and probably even for overall system efficiency (with battery loses accounted for) depending on the ratio of city to highway driving.