To say something under the rose means to say it privately or confidentially, but in this context it is implied that whatever is said publicly must remain a secret.
Context: Attendees understood that whatever was said publicly or openly - or sub rosa - had to remain a secret.
Hypothesis: Attendees understood that whatever was said under the rose - or sub rosa - had to remain a secret.

Given this context, determine if the hypothesis is true (Yes, no, or unknown)?