Fey, Xeno was an idiot. The paradox of the runner and the tortoise only works as a paradox if both contestants are moving at exactly the same speed, and the paradox Reed is citing presumes that eventually the runner would have to cross half of Nothing, a value which is inherently indivisible.
Thank Grodd for Paul Cornell, one of the few current Marvel writers who isn't mediocre at his best and utterley useless at his worst. It's rare to see a character so malevolent and yet so touchingly endearing. But then again, we all fell in love with the neck-snapping Warren Ellis-sharpened Norman, back before he became Dick-Cheney-On-Drugs-In-A-Tin-Can.