Tobi can't OHKO, unless he can born again as a hyuuga =D
On a more serious note, why would Madara show off? Deidara surely could do the service, and his "tobi" character speak a lot of gibberish usually, so he could be lying. My guess is that Deidara killed it.
He was capable of blowing up a whole city with one shot, so I don't see why he's not capable of squashing a crazed, little monster. Until the story proves otherwise, it's just your opinion against mine.