I don't think it's harsh at all. Such an epistemology contributes nothing but false positives because parties with conflicting interests agreeing has zero bearing on truth. It's not even a good rule of thumb; it's just completely useless and irrelevant to knowledge production. Most people believe in some kind of religion or other nonsense like astrology; that they disagree on other things yet agree on that has absolute no bearing on the truth of any of those things. Think about it.