No need to just look at the PPv numbers
What PPV numbers? The only time he's said that line to build up a PPV was for... I mean IS for the Royal Rumble, which hasn't happened yet. Granted you're right that making your opponent seem as big a deal as possible is the best way to sell a PPV. Rock's doing this, and Punk did it for Rock on Raw as well. It's just neither guy is gonna put the other over himself, which makes sense, and is why posts like TheFlBOB's in this thread are stupid. This feud isn't one-sided. Rock isn't putting Punk over on the mic because he has no credibility. He's doing it because that's what sells the PPV.
And to counter a point TheF1BOB brought up, mentioning how Rock didn't have to put over guys like Austin, HHH, Mankind, Taker, Big Show, Cena, etc., that's true. He didn't. It's a moot point though because he doesn't have to do it for Punk. However the difference is for all of those guys except Cena, it was a different Rock they were facing that made his name off of grinding his opponents into dust verbally in hilarious fashion. And for Cena, while I can't say for certain, I don't recall Rock ever ignoring Cena's talent, committment, and (kayfabe) resilience, but spent most of the time countering Cena and making jokes that didn't mean anything aside from a quick laugh. Worst thing I can remember from Rock against Cena was when he was talking to that kid (who was imitating Cena) and said "You took the ball and you ran with it! You didn't run really far, but you ran your little legs off!" or something like that.
But yeah, this isn't some one-sided build, Punk did have credibility even before The Rock opened his mouth despite what haters like to believe, and they're building this up perfectly.