Except Voltec was also able to run the ICE at its most efficient setting by switching to Serial hybrid mode, where it was acting like the e-Power mode.

Voltec was using clutches to let you run like either the e-Power example or the Prius example on the fly, or even somewhere in between with the 2nd generation depending on power demands: https://gmauthority.com/blog/2015/02/secrets-of-the-2016-che...

CS3 is what you're describing when talking about the most efficient ICE operation