I'm not requiring you to prove a negative, but a positive: that people are capable of reliably maintaining neutrality and keeping a good enough grasp on a wide variety real world actions that they can judge them fairly.
At what point did I say "100% biased"? That'd be the strong claim if I did.
But I didn't. I said everyone contains biases, misunderstandings and other things that prevent them from being a truly neutral and accurate arbiter. You could have argued that isn't strictly necessary (as it isn't in the case you're presenting there), but you didn't; you wanted to argue it wasn't true, because the degree of fairness required when you're both being an arbiter and being the person doing part of the opposition, as a typical RPG GM does, is a considerably higher bar.
The real issues is that there's a fair number of people who think there are benefits to a more ruling-oriented playstyle that gets lost as that gets diminished. They have a right to that view; there are circumstances that can even support it. But they don't get to take it as a given, and trying to claim there's never or rarely any problems that can come up with that only makes no sense, and people don't get to just assume it as a default premise as is often the case with some old-school approaches.
So yeah, be as outraged as you want about this, but I don't think I'm the one making an extreme claim here.