At cruise, yes. However during takeoff and go around they run at higher RPM to develop more thrust. Using batteries for takeoff and only using fuel for cruise or backup power is smart.
I think it's even more than that. The hybrid engine has to run one single RPM to drive a generator, it can be vibration insulated, probably needs to be only smaller because it doesn't have to provide the high max power the conventional motors have etc.
It's why I believe hybrid cars that use a gas motor only to charge the batteries are more reliable and more efficient.
RPM is one thing, but the other is that engines run more efficiently at altitude due to the temperatures being much lower there.