1. The hoax authors didn't submit gibberish or anything, like with Sokal - they submitted coherent papers that they considered absurd, but which they expected the journals to find reasonable. As such, they only thing they can even pretend to have proven is that they and the journals disagree about what's reasonable.
2. IIRC, every one of the initial hoax papers was flat-out rejected! If the authors had any backbone they could have stopped right there, and published a paper called "These grievance journals aren't as bad as we expected." :D But they didn't - they set out to iterate on each paper until a journal accepted it. And if your experimental methodology is "keep trying different things until X happens", you can hardly pretend to have proved something when X happens.
I mean, I agree with the premise the hoax authors were trying to prove. But their "experiment" was silly, and I think it's dishonest of them to pretend it proved something. Meh. Meh!