Huh. I've had dreams "in Russian", Greek, Latin, Japanese, and Arabic. I know none of these languages beyond a little counting or a few words, but they seemed very real at the time!
I still think that being able to crack an improvised joke based on the situation one is in is a stronger sign of fluency. But it rests on similar assumptions: having internalized the basic structures of the language to the point where you can spend your cognitive resources on execution.
The first thing I learned from a second human language teacher was that I'd know when I was becoming fluent-ish when I had a dream in Spanish. At least for second-language teachers, internalizing concepts and not thinking about them is a sign of learning progress. It doesn't seem so disingenuous to me, having internalized a lot of things that felt unfamiliar before and having experienced proficiency improvements as unfamiliarity melted away.