When Ponyboy says this, he does not mean it in the sense that you can never physically win a fight against them. He is not saying that you can't beat up one of the Socs. What he is saying is that no matter what you do to them, they are still going to end up on top in society.
Look at the whole passage where Pony says that you can't win against the Socs (it's on p. 11) of my book. He says that it is
because they've got all the breaks and even whipping them isn't going to change that fact.
So what Pony is saying is that even if you beat up a Soc, they're still pretty rich and they'll still have nicer stuff than you and they'll still grow up and go to college. You, meanwhile, will always be poorer and have fewer opportunities. So no matter what, they win.