And that 1W difference probably comes from mass storage access. I'm pretty sure power management was in one of the Linkedin profiles I've found a while ago, so my guess is that it simply isn't enabled in the current firmware.
It's likely that "power management" in that context is simply related to reducing max power consumption. Hardware techniques to reduce power consumption during periods of low load (like power gating) aren't really things which can be turned on or off by firmware. My guess is that they decided against significant power gating for the GPU die, given that it increases the die size and reduces performance (although only very slightly) and the GPU die is already pretty big and power efficient in the first place.