An assumption that is, more or less, correct. The attendance numbers for MK are much higher than the other three parks, which makes it true. Are there exceptions? Sure. But the in the vast majority of cases where the 'only one day' scenario exists, it is true that that one day would be at MK.
He said the Fantasmic theater, not BatB. I don't think its feasible because of the setup of the Fantasmic stage and little room behind the stage, but I'm sure its something they've looked at - reusing the F! stage for other things during the day.