That's the first mention I've seen of it being a baker's name.

Ciardi, on the horse and fart etymologies, supports both American Heritage and Random House, already cited:

"[Ger. /pumpern/, to fart; /Nickel/, devil (Old Nick). Because this bread is coarse (sometimes wonderfully so) but was said to be so hard to digest that it would make the devil fart. (The story that Napoleon, retreating from Moscow on his horse Nichol, fed it this bread, calling it /pommes pour Nichol/, applies for Nicholas, but is not an etymology but a strained joke.)]"