How can you tell if a short person is slouching? Or a tall person?

I'm not the author, but I assume it benchmarks the highest height of your head, blurs from there, and updates its baseline if you ever appear higher.

Meaning that the way to have "perfect posture" is never to sit up straight in the first place :-)

If you assume a person’s chair height and desk height are both set optimally, then I guess the person’s height doesn’t matter for this detection.