[GrantS]

He's using the quickness with which we've progressed to VR as an existence proof that creating universes is not as hard as once expected, such that "universes" might actually be numerous and commonplace. From there, it's a statistical argument -- if "existence" contains one billion "universes" and only one of them is the base level of reality in which all the other simulations are nested, then if we assume a uniform distribution over universes, we have a one in a billion chance of being in the base level of reality.

That's a very simple version of the argument with obvious flaws, but that's what he meant. Nick Bostrom has made a much more elaborate version of the argument.