Exactly, if you take away the USO. However, Del Potro did win that US Open beating Federer and Nadal on the way, so there's no reason why it shouldn't be counted. That weekend alone eclipsed Ferrer's entire career, both in terms of level of play and achievements. Even with a limp wrist, JMDP is still easily better than Ferrer.
The context of that win was very dodgy to say the least. Nadal had an abdominal tear - he shouldn't have been in SF in the first place, while Noserer was choking left and right, when was the last time you saw him DF in double digits and serve at ~50% in a slam? He was still owning the ballbasher even playing like shit until the choke came
Honestly, as a Fed hater, I can say that USO 2009 was the biggest waste of his career. If he didn't inexplicably choke like a mug, he would be sitting on 18 at the moment. Ferrer and many other players were never afforded an opportunity to have the stars align for them like Delpo. That's why he's a fluke. He never backed up his results, he just got hot and lucky for 2 weeks. At least Mandy made multiple slam finals until USTA gifted him the USO