No, they have a "battery budget". It the CPU power draw goes down that means the budget goes up and you can spend it on other things, like a nicer display or some other feature.
When you say "up to 10 hours" most people will think "oh nice that's an entire day" and be fine with it. It's what they're used to.
Turning that into 12 hours might be possible but are the tradeoffs worth it? Will enough people buy the device because of the +2 hour battery life? Can you market that effectively? Or will putting in a nicer fancy display cause more people to buy it?
We'll never get significant battery life improvements because of this, sadly.
The OLED likely adds a fair bit of draw; they're generally somewhat more power-hungry than LCDs these days, assuming like-for-like brightness. Realistically, this will be the case until MicroLEDs are available for non-completely-silly money.
OLED will use less for a screen of black and LCD will use less for a screen of white. Now, take whatever average of what content is on the screen and for you, it may be better or may be worse.
White background document editing, etc., will be worse, and this is rather common.
It's not weird when you consider that browsing the web or watching videos has the CPU idle or near enough, so 95% of the power draw is from the display and radios.
