Having watched all of both of their fights? Rogan has said it multiple times. He might have even been the one to coin the phrase.
"This mean-mugging thug from Stockton can back up everything he says, and then some. A pioneer of a boxing style referred to many commentators as the “Stockton Slap”, Diaz opts for a large volume of strikes over power in the stand-up game."
https://connerstoesser.wordpress.com/tag/stockton-slap/
"Nate, along with his brother
Nick Diaz, possesses one of the most unique boxing styles in any martial art. Known as the "Stockton slap," Diaz will plod toward his opponent flicking out a high volume of punches, rarely throwing at full speed or power. This style takes advantage of his height and reach, as he can start his combination outside of his opponents' range before stepping in with harder shots."
http://www.mmamania.com/2013/11/26/...-breakdown-nate-diaz-strengths-weaknesses-mma
It gets confused a lot because they literally slap fighters in fights, but that's just a bitch slap. The term "Stockton Slap" actually refers to their boxing style.