This is true.
I've done this test previously, where I went from USB to Optical to just the aux input.
Both the optical and aux input has the same time where the battery would run out after 8 hours (a far cry from the 15 hours rated by FiiO). On USB, my E17 ran out after 23 hours believe it or not and this was with continuos usage. If I was using the E17 normally with USB as in a few hours a day, it could last for weeks even.