Because it says after Ancient Power is learned, but Piloswine only learns it as a level 1 move, so do you have to use the move relearner?
Since you've stated that this is for Platinum, I'm editing this.
A Swinub can get Ancient Power by breeding a member of it's family with a Dunsparce or Kecleon that knows the move, as well as through move relearner. Dunsparce learns it at level 41 in Platinum, and Kecleon at 55 or by a move tutor.
Hope I helped!
You can find wild Mamoswines in Giant Chasm, Stony Wilderness, and Slippery Slope. You don't need to use the move reminder to catch wild Mamoswines.