Robertson doesn't strike me as a particularly scholarly thinker, but even less well-thought religious folk have confronted the problems of evil and tragedy. The story of Job is a common subject of discussion in churches and among religious folk, and it's always framed as horrible things happened to Job because of his belief in a deity and because of the deity. Christians aren't unused to the concept of bad things happened because of their faith rebounding on them.

He's fantasizing about the outside world giving 'indisputable proof' of external morality. The religious folk have /countless/ scenarios like this, and the better-spoken ones will explicitly call them tests of 'relative' morality.

There's a pretty easy response to Robertson's thought experiment even within that framing -- to borrow from Babylon 5's Marcus Cole, "wouldn't it be much worse if life were fair, and all the terrible things that happen to us come because we actually deserve them?" -- but the state of promoted discussion by atheists is so terrible that Robertson's probably not aware of it.