Outright calling it a delusion, straight in the title no less, really isn't the tack I would have taken. I also feel there is a kind of ableism and danger in immediately leaping to calling other people mentally ill when they do something we find wrong or dislikeable. Someone can be profoundly screwed up - engage in mass shootings even - and technically not be mentally ill, unless we are going to start calling all soldiers willing to mass shoot people mentally ill. 'Normal' human psychology is perfectly dangerous on its own.

That said, it does a good job at shocking people, getting talked about more than more tame offerings, and it has apparently made some people deconvert, and it is a genuinely good question why we should arbitrarily consider someone deluded in one case where they hear voices and yet not so in another case.