Yeah, Del Potro has to get past Ferrer to win a Slam. Oh wait, he doesn't, he's not a potential Slam winner but an actual Slam winner despite his young age and wrist surgery that threatened his career and sidelined him for a year. And Berdych and Tsonga (the other guys he mentions) have both played a Slam final despite being far younger than Ferrer, who never played one. In the rare occasions (3 in fact) where he reached semis, he was just a bye to the top player he faced, didn't even put up a fight.
Well, Del Potro has met Ferrer twice in a Slam - and lost. So, when they meet in a Slam, Ferrer wins. Del Potro can't get past him. Berdych, we don't know, but the win-loss record suggests Ferrer would be the winner.
Tsonga has beaten Ferrer once in a slam, but their rankings position is almost equal.
So in conclusion, I was not talking about how far any of them can go, I was talking about getting past Ferrer if they met in a slam. DelPo, so far, is not up to the task.