I’m not sure how helpful this will be, but I think it is misleading for you to think of these things as “rules”. Calling them rules implies that people know what they are, and try to abide by them. But I don’t think thats whats happening.
Since written communication conveys so much less emotion than verbal communication, people tend to read more into the textual form to infer things like emotion or intent. There aren’t specific rules for this, people just pick up on patterns. For example angry or excited people tend to use capitals to “sound louder”. People talking casually tend to leave out some grammar. Friendly conversations tend to have more long rambling sentences, as opposed to someone trying to prove a point with concise, well punctuated sentences.
So I think what NTs are doing is just subconsciously figuring out these patterns to determine a likely emotion/intent behind messages.
To me, “K” isn’t rude for the reasons your friend tried to list, but it does sound “colder” to me, probably because my brain sees a pattern where the people who are not interested in talking to me will tend to use short succinct responses, while people happy to talk will use longer form.
But a huge part of communication, especially online, is just getting used to the people you talk to. It’s happened many times that someone who seemed grumpy to me at first turned out to be very friendly, etc.
From twitter or like, from the human species?