This is not true, for the same reason you can't bake a batch of cookies at 2300 degrees for 1 minute instead of 230 degrees for 10 minutes. I imagine delivering the amount of heat required to bake a pizza in the microseconds of a nuclear explosion would vaporize a substantial part of the explosion-facing side of the pizza while leaving the back frozen.
I think spinning the pizza at relativistic speeds only causes more problems. Household microwaves run at 2.45 GHz, and at 2.45 billion revolutions per second a 3.9cm diameter pizza would have an edge velocity of the speed of light (ignoring relativity, which I'm sure does not make things better).
If you're thinking of the neutron and gamma radiation emitted, that happens on an even shorter time scale than the thermal radiation (nanoseconds IIRC) and is absorbed poorly in comparison by a pizza.