What does he expect "believers" to say to him? He mocked their belief in something, can they mock his belief in...nothing? No, they're going to push on him what they believe are the consequences of his actions, eternal damnation.
Since I came to the US, I've found atheists mock Christianity but not other major world religions. Why is that? Is it because that is the country's largest followed religion?
Oh, I suspect that that's pretty much exactly what he wants. :-) Well, that and to make people laugh, since he is a former stand-up comedian, nowadays a writer.
Maybe. Might also be because they are most familiar with it, or some of them might be cultural relativists to some degree, meaning that they might see the concept of religion silly in modern western culture, but are less judgmental when it comes to cultures of other kind.
Or they don't see other religions as 'their business', or they simply don't see a point in mocking other religions in a place where there are not that many believers of the said religions to listen to them.