It's not completely wrong in hindsight†. It's only "wrong" if you assume "app" means Dropbox itself, and if you ignore the context of the thread. Brandon was talking about an application that framed Dropbox as an alternative to USB drives, and critiquing the pitch.

There was a time on HN where it actually made sense to bat a YC application back and forth, and to talk about what might make a more or less compelling one. It's long gone, and the people dunking on this comment have completely lost touch with what the original community was. I'm not saying that community was better (it was much more insular), but it was certainly more collegial.

† In fact, I think we now know it was literally correct, and YC had the same qualms, accepting Drew Houston just to get access to him and apparently hoping he'd find something better to work on.