"Shouted him to pieces!" Makes for a more dramatic story than "shouted and knocked him down, then ran him through with a blade." Plus if you've ever been subjected to the game "telephone" you know that accuracy deteriorates quite rapidly in retelling. A lot of the npcs seem to enjoy hyperbole. Just like a lot of people in real life.

Aside: Not relevant to Ulfric but isn't there some Black Book of Hermaerus Mora that turns fus ro dah into a literal shouting to pieces power (disintegration effect)? I suspect because at least a couple people said, "damn I wanna shout someone to pieces, make it happen!"