"Better battery life"
How do they know? If it has a quad core instead of a dual core, that would probably take more power. I would imagine a Retina Display takes more power on its own. 2x the pixels means a lot more work for the GPU as well.
Assuming all the above is true like a normal computer, then you would need a bigger battery just to obtain the same battery life as the previous. Bigger battery =/= more battery life in this case if you look at hardware only then.
That being said I wouldn't put it past Apple to increase the battery life still. Maybe the CPU/GPU/Screen don't take much more because they've improved hardware efficiency, they increased the size of the battery by more than enough to compensate, and/or have worked a lot on software, but yeah.