I'm not sure either. My guess is that since the pinky means pinky promise, he meant that the MC and prince were promised to each other, as in going to get married?
Thumb means boyfriend, and pinky means girlfriend in Japan, if i remember correctly orsin
You're right! I read the explanation in another manga with the pinky and then searched it up myself. Showing the pinky is a discrete way to ask a guy if the girl he's with is his gf/wife/lover/etc.
waah iwant moreee