I'm confused how this is even supposed to demonstrating "metacognition" or whatever? It's not discussing its own thought process or demonstrating awareness of its own internal state, it just said "this sentence might have been added to see if I was paying attention." Am I missing something here? Is it just that it said "I... paying attention"?

This is a thing humans already do sometimes in real life and discuss -- when I was in middle school, I'd sometimes put the word "banana" randomly into the middle of my essays to see if the teacher noticed -- so pardon me if I assume the LLM is doing this by the same means it does literally everything else, i.e. mimicking a human phrasing about a situation that occurred, rather than suddenly developing radical new capabilities that it has never demonstrated before even in situations where those would be useful.