My understanding is that oleds are a weird beast. Since there’s no backlight, each pixel can be considered a small colored light, if you have a fully black screen, then it’s essentially off and not using any power. However, there are instances where the peak brightness is limited to a small portion of the screen, because blasting the entire thing of full brightness white would pass the power supply capacity…
That said, let me stress this: it’s my understanding. Not a hard fact, I might be wrong or just basing things on old information.
If you have other ways to play a game, consider buying it regardless of the rating for the Steam Deck. Sometimes verified games update in a way that makes them way too hardware intensive, others might actually be playable regardless of what they say and the only real way to find out, is to try. For example, I wanted to try it so I setup Steam VR on the Deck, added ALVR, set it to minimum resolution and fps… I mean, Taskmaster VR worked. I had to make the resolution inside Steam VR all the way down, and it keeps a shaky 60 fps (doesn’t bother me, others could get motion sickness) but it was playable. Obviously it was docked, so 100% just curiously as on the other side of the desk there’s my actual gaming computer, but…