Okay, so I'm no scientist and I don't know the inner and outer workings of a bomb, but even supposing he was able to let the time bomb go and somehow fly at a super speed (which by the way, to my memory, was not even shown with his powers when Robin easily incapacitated him.) I sincerely doubt he could get away to a safe enough distance where only part of the explosion would graze him. And even if that was Oda's intention, the fact he never displayed his supposed super speed earlier in the arc and the fact that he's leaving too much to assumptions which is still bad writing no matter how you slice it.
Flying. Flying. Fly fly fly.
..and hey, how quick do you think it was for Pell from finding Luffy, taking Luffy to get some meat then getting from Rainbase to the capital, Alubarna?