A delusion is a strongly-held belief or conviction which ignores all the powerful evidence against it. If you believe in God you have chosen to believe something for which there is no evidence for and a massive amount of evidence against. The word "delusion" for believers in God is, therefore, perfectly valid, though, admittedly, provocative. But you can't really address the matter of believers getting it wrong without being provocative. I think there is nothing wrong with his use of the word. Of course, if you mistakenly hold to the view that "delusion" can have only a medical meaning, you would disagree. But you'd be deluded in sticking to that, as the use of the word in its non-medical sense is widespread and accepted.