I saw some parts of this video, so I'm not sure it was mentioned, but the key parameter that is optimized at all times in a hybrid is brake specific fuel consumption:

https://en.wikipedia.org/wiki/Brake-specific_fuel_consumptio...

This is possible (and done) in any automatic, but in hybrids it was taken to the extreme, with electric motors covering for situations when either the RPM or torque are not where desired.

In fact, my experience is that during highway driving it's actually slightly off the optimal point, charging the battery in the process. I speculate that it's so that the surplus power can be released immediately by just making the electric motor stop acting as a generator and go into, well, motor mode.