My laptop consumes most of its power via the discrete GPU, display and other peripherals. I know, since if I switch from the good GPU to the crappy one, and dim the screen, my battery life goes from 2.5 to 5 hours. Thus, with everything on, at least half the power draw can't be CPU, so there is no way this could double my battery life.
Math tip: A 50% increase in battery life (what they actually claimed) isn't the same as doubling it.
Also, since a big selling point for Haswell (aside from power efficiencies) is the claimed greatly improved (~2x for laptop-oriented models, ~3x for desktop-oriented models) improvement in graphics performance, I'd be very surprised if their claims for about battery life were focussed on systems using discrete GPUs rather than relying on the integrated graphics on Haswell.
But, when idle, with a dimmed or off screen, yes, it could help a lot. Apparently Haswell has some major improvements to handling network (including wifi) packets when in low power modes, and this should be a big win. I assume this kind of situation is where they got the 2x number
Well, except that they explicitly claimed that was overall battery life, and it was a 50% increase not 2x, and they actually cited numbers for improvement in idle life and it was much higher than the +50% claimed overall (or even the 2x you pulled out of who-knows-where), since their claimed idle-mode improvement was twenty times (TFA is less clear on this, but Computerworld covers the same event with more specificity: "And in idle or standby mode the chips will do even better, extending battery life by up to 20 times, [Rani Borkar, Intel's Architecture Group VP] said." [emphasis added])