The efficiency of the entire system is more important than the efficiency of the engine itself. Charging the battery and then discharging it also incurs losses. Many series hybrids will prioritize generating electricity when some of it is also required to directly drive the motor. They usually don't sit around at low-load situations just generating electricity, unless SOC is low.

> The gas engine has maximum efficiency at about 80% throttle.

ICE efficiency varies in multiple dimensions based on load and RPM, and in a series hybrid, you have some ability to dynamically influence these... throttle would be one of those inputs.