Just because people would rather say "it's a quirk of the definition" than try to give a formal lecture on thermodynamics in a comment section, doesn't make it "nonsense." These are complicated concepts when taken beyond the layman level (and atomic gases at negative Kelvin values is well past the layman level).

Many times such questions don't have easy or quick answers. People around here are happy to give a basic rundown of concepts, or a detailed discussion of a specific point. It's rarely worth the time and energy to try to give someone an introduction into an entire field of science, however. The Wikipedia entry on "Temperature" is over 10,000 words.

But just because a concept can't be easily explained in a website's comment section, that doesn't make in "complete counterintuitive nonsense." It just means it's a complicated concept. If you tried to redefine every concept that couldn't be explained to a layman in a reddit comment, you'd have to throw out huge swaths of our current understanding of the universe.