Sightings:

• Arguments that aren't actually arguments: argument by tribal affiliation was certainly in full force, as well as a certain general condescension bordering on insult.
• Statistical illiteracy: in an only minor variant of your hypothetical exchange, I said that very few people are doing too much exercise (tacitly, relative to the number of people who are doing too little), to which someone replied that they had once overtrained to their detriment, as if this disproved my point.

I was also struck by how weird it was that people were nitpicking totally incidental parts of my post, which, even if granted, didn't actually deduct from the essence of what I was saying. This seemed like a sort of "argument by attrition", or even just a way of saying "go away; we can tell you're not one of us."

A general pattern I've noticed: when processing an argument to which they are hostile, people often parse generalizations as unsympathetically as they can. General statements which would ordinarily pass without a second thought are taken as absolutes and then "disproved" by citations of noncentral examples and weird edge cases. I think this is pretty bad faith, and it seems common enough. Do we have a name for it? (I have to stop myself doing it sometimes.)

Your symbolic arguments made me laugh.