I think he isn't being clear with his terminology.  In order to overclock an Android device, you have to root it.  It isn't really the overclocking that (potentially) benefits battery life, it is gaining root access and altering voltage based on clock speeds.  Most of the time you don't use the CPU at max clock speed, whether it is overclocked or not.  So, you can lower the voltage and see little or no adverse effect.  Well, sometimes.  I've rooted an older Android phone, and I ended up just letting an app change my voltage.  It was just too much trial and error for me, and I ran into stability problems a lot trying to find the floor, but if you were more patient than me, you could theoretically achieve near perfect voltage settings.  If you rooted a device and installed a custom kernel, the voltage settings are probably already changed.  I saw a slight gain in battery life, but nothing huge.  I was always fairly careful about maximizing battery life anyway, so that might have kept my improvement to a lower level than a lot of people.