So if more and more bots pass the test, this can be the result either of an increase in the bots’ Artificial Intelligence, or of an increase in humans’ Natural Stupidity.

Or it "simply" plays with human biases, which are very natural. Stuff like seeing faces in everything that somewhat resembles two eyes and a mouth (or sometimes just the eyes and a head like shape etc.) is pretty hard wired. We have similar biases in regards to language. If something reads like it was written by a human, we immediately sympathize with it. Which is also the reason these LLMs are so successful and cause so many people to fear our AI overlords are right around the corner. Simply because the language is good we go into "damn, that's like a human"-mode.