Wrong. The power item only increases EVs on the mon holding it. All other mons with Exp Share will gain whatever amount of EVs the defeated mons give.
Take, for example, a horde of Wingull. Each gives 1 Speed EV, for a total of 5 Speed EVs if you defeat them all. Now, you have, say. a Greninja battling, with a Talonflame in your party. The EXP Share is on. Greninja is holding the Power Anklet (+4 Speed EVs). After Greninja Surfs to kill all the Wingull, your Greninja will now gain 1 + 4 = 5 EVs times 5 for a total of 25 Speed EVs. Your Talonflame, however, since it was not holding any Power item, will gain only 5 Speed EVs.