I don't see how that is supposed to be relevant to the article's claim that grain adds authenticity and realism. Someone claiming that non-diegetic music adds to authenticity would also be weird.
I think what the author meant is that film grain is part of the authentic visual aesthetic of cinema produced using celluloid materials. A de-noised image is therefore inauthentic as that is contrary to the appearance of the original projected image or source/master elements.
