Does John Gruber have any demonstrable evidence to back up his opinions? No, they're rank speculation. They are, at best, educated guesses. I tire of this kind of blogging. I'm not saying that Mr. Gruber isn't informed, or intelligent, or perceptive. What I'm saying is that he's got nothing beyond that, and that's not good enough for me.
I think your reductio would work better if you used non-fiction: The Republic, The Nicomachean Ethics, The Meditations on First Philosophy and Leviathan are full of opinion and rank speculation. At best, educated guesses.
And, in fairness, how much of what's in any of those would get much agreement these days? The Republic advocates the abolition of families, a ban on poetry, and a systematic campaign of deception on the part of the ruling classes. The Meditations contain what despite stiff competition is still one of the worst ever attempts to prove the existence of God, and the whole "assume nothing and work up from first principles approach" is pretty much completely discredited. Leviathan is a sustained argument for totalitarianism.
I suppose the Nicomachean Ethics might get a bit more love, though it's a lot more self-centred than you might expect from the title. (Aristotle's basic question is "how should one live so as to attain eudaimonia?", and that last term means something much more like "happiness" or "well-being" than, say, "virtue".)
Which isn't to say that those works aren't historically important, or interesting, or produced by first-rate thinkers. But it does kinda suggest that if what you're after is truth, opinion and rank speculation aren't very reliable ways to get that even when done by geniuses.
(Yes, "kinda suggest" rather than "prove" or anything similarly strong; it is of course possible that we're all wrong nowadays and that, say, Descartes was right. But it's not looking likely.)
