You're right, they have done a ton of work. Unfortunately, you can't make developers use it. So you find some games that don't seem to do too much but waste your battery, and others that are very impressive and barely touch it.
I'm looking at you Pokémon GO.
Correct. If it lasted 90 minutes they would clearly attribute it to playing the game. If however a regular mix of typical daily activities gets low battery life, that's just the phone's fault.
But I would add that Apple has done a lot of work to make their game APIs power efficient. Metal is a perfect example of a more proprietary approach that achieved significant speed and battery life gains.