Yes it is indeed true. Mobs gain more dps per AP than a player does. So inversely, if you use Imp demo shout, you will be dropping that mobs dps by a substantial amount.
I have tested and does work. My current warrior is leveling 2h fury and I find the 5 points in Imp demo shout and Imp battle shout too good to pass up while leveling. In a group/raid environment only a select few will need to spec into these so all receive the benefit.