For it to be a mathematical fact, it needs a mathematical proof. Go ahead...!
Like it or not, rationality is not mathematics—it is full of estimations, assumptions, objective decisions, and wishful thinking. Thus, a "theorem" in evidence evaluation is not a mathematical theorem, obtained using unambiguous formal logic.
If what you mean to say is that Aumann's Agreement "Theorem" is a fundamental building block of your particular flavor of rational thinking, then what this means is simply that I don't fully subscribe to your particular flavor of rational thinking. Nothing (mathematics nearly excepted) is "true, full stop." Remember? 1 is not a probability. That one's even more "true, full stop" than Aumann's ideas about rational disagreement.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.