>> Why are people who have little or no firsthand knowledge of the Middle Ages absolutely convinced they know the facts on this issue?

I used to have these arguments with an older relative, some acute, some chronic, about a) whether the edible part of an urchin is eggs or genitals [1], b) whether urchins with little pebbles and bits of seaweed on them are males, ornamented thusly to attract females [2], c) whether cypress tree sex can be determined by how open wide are their branches [3], d) whether the ruins discovered by Heinrich Schleimann on the coast of Asia Minor are really the ruins of the mythical Troy [4], and, e) whether ascent blackout during free-diving is a thing or not [5].

I've given up. People know what they know, either because their mother told them so when they were young, or because everyone knows, or because they know better than you. If someone's made up their mind that they're right and you're wrong, then they're right, you're wrong and you can't change their mind.

_______________

[1] Genitals.

[2] No, sea urchins do not have eyes.

[3] No, cypress trees have both male and female parts.

[4] Undetermined.

[5] It is.

Sometime you may find this person has stealthily changed his or her mind, and now always thought the other thing, and if you say they previously thought the opposite, you're wrong about that.