I think it's obvious enough without getting into the weeds like this unless their sample size is unreasonably small. E-book yes obviously counts. For the rest, like fanfic? Probably, but does it matter here? Is this actually going to move the needle?
Be cool if they would research that, instead of asking rhetorical questions and assuming that none of it matters and everything is "obvious."
I think it's just people being snooty. Bestsellers are trash, and by definition a plurality of readers are reading them. I don't think someone who reads Gladwell has any greater cognitive power than somebody who reads twitter.