Roxy,
The manual does say that - no doubt about it. But like most modes if you use a higher resistance it does not harm anything it you go really high with the coil resistance you just won't be able to reach the full 60 watts output. If you look up the specs for the chip-set as a separate item they are considerably broader.
But I would guess you don't use it anywhere near the 60 watt level anyway. So if you put a 0.5 ohm coil on yours and get only get to (lets say) 50 watts would it really effect you in any way probably not unless you are entered into a "Cloud Chaser Contest".