Puzo probably intended the reference as an example of Vito having a sharp eye for personnel, despite his recent disability and apparent surrender to the other Dons.

Recall that, when Clemenza gave Rocco the assignment to whack Paulie, he told him that he'd get a reward down the line, but that the family was preoccupied with the current crisis right then. Presumably he got closer to Clemenza after that. But Vito was in no position to notice that, so perhaps he was unaware that Clemenza was already recognizing and rewarding Rocco.

Two interesting sidelights:
--Clemenza was the one who brought Neri to Michael's attention. And, as it turned out, Neri, not Rocco, was Michael's man.
--In the film, just a few days (maybe even a few hours) after Rocco makes his bones on Paulie, he's sitting with Sonny, Tom, Michael, Tessio and Clemenza, scarfing down Chinese food with the mighty and participating in the plan to whack Sol and Mac.