I'm guessing he hasn't. More likely he's going with what he hears from Quora or some other reputable source of anecdotal non-evidence.
I actually couldn't give a flying fuck about Satanism, and if Islam had the same numbers, I wouldn't give a fuck about that either. I only bring it up because it's clearly in stark contrast to Christianity, which is the dominant religion in both UK and USA, therefore our nations have what could be considered Christian values.