Now that this has turned into a discussion about statistics, it's more important than politeness.

any insult by prior probability of the form: "you're probably about average quality for a poster, because one post isn't enough to prove otherwise."

What is insulting is what you are choosing to privilege. There are all sorts of things that are true about someone. For example, I am ethnically a Russian male. A fact about Russian males is that they have a very low average life expectancy (for obvious reasons). We could privilege this fact and (in a discussion about cryionics say) point out that, without additional evidence, I am likely far closer to death than an ethnic American my age. And because of this I should consider signing up for cryonics more seriously than an American my age.

This would be true (in fact not about me, because I almost never drink, but the "statistical reasoning" is sound), but an extremely socially stupid thing to say. Knowing what true things to privilege is the difference between Leonard and Sheldon on Big Bang Theory.

"Regression to the mean" as used above is basically using a technical term to call someone stupid. These sorts of "fact reporting" events don't exist in isolation but in a larger social context, where people might use them to assert dominance and all sorts of other things.

I think your report that "this" is leaving a bad taste in your mouth is extremely fascinating to me.