I dont think National Geographic would lie ...
This pic ... (fucking great pic by the way) shows a similar "slim seal" (i.e decoy) being used to trigger an attack.
At the same time there are tons of pics and videos out there showing real attacks.
If you wanna see something realy cool try and find that video that shows about 20 great white sharks eating the floating corpse of a whale. I think that was from National Geographic and was shot in the same area as this. They ate the whole thing in a day.