I guess the idea is that even though our observations are at least a billion times below the Planck scale, the propagation times are very long and the difference in arrival times could be measurable even if there is just a hint of Planck-scale influence. It's provocative, that's for sure. What bothers me about it is that if you are a billion Bohr radii from a proton that's like a centimeter, so this is like trying to probe the nature of a hydrogen atom by watching electrons that pass a centimeter away from the proton! How do we know how much physics we're missing there? Still, I realize that any confirmed depature from the equivalence principle is probably a Nobel prize.