I've been on the internet since the early 90:ies, and this does happen semi-regularly, especially during the last decade. But I have never in my life experienced such situations stemming from an agreeing reflection/interjection during face-to-face communication. Sometimes it feels like people are (un)intentionally looking for reasons to disagree rather than anything else.
I agree with them, your original post lacked clarity. I propose that the reason these types of conversations are less likely in person is because there is typically no log of exactly what was said and people tend to get defensive and narratives change. This makes it a pointless endeavor.
I would suggest, rather than wondering why people on the internet point things like this out, maybe wonder how many people in real life never bothered and just write you off.
