The author's conclusion is flawed: paper and ink are never truly pure white or black. Perfect white/black exists only in digital spaces, while physical materials always have subtle variations in tone and color.

I think that's sort of what the author is getting at, if expressing imperfectly -- when you look at black ink on white paper, you're almost certainly not seeing a physical embodiment of #000000 on #FFFFFF. I tend to think moving at least one of the two colors off "pure", if not both, makes pages look more pleasant. (To stave off people leaping in to say "you web designers hate contrast," I'm not talking about moving to light gray on slightly less gray; it's not difficult to keep well above the WCAG's 7:1 AAA-level contrast ratio while bringing things down just a notch.)