To be clear, the author IS proposing that there MAY be negative mass particles, not that this is all mathematical. That statement was simply a tip of the hat to the fact that none have yet been detected and its possible his theory is just a "accurate way to model" dark matter/+dark energy. It's just a nice way saying "My thoery might be the Newtonian Mechanics of Dark Matter/Energy, close but possibly wrong."

Why crap on the fact that this may lead to a warp drive.

>Or is there something about the nature of the dark fluid hypothesized that would make it useless for something like that?

Assuming it was, literally, some kind of magical FTL juice... it all seems to be outside the galaxy, which means tens of thousands of years' travel just to get to it. If we're willing and able to do that, why bother with FTL at all?