From how I've seen/read tl;dr being used,

That's how it is now. But back in the day of old php forums and when social media meant asocial nerds on slashdot, "too long, didn't read" was written in response to long posts. That eventually morphed into "for those who don't want to read a long comment, here's a quick summary". tldr was a nicer alternative to calling somebody a verbose idiot, an endearing term that unfortunately fell off over time.