How can I not get a joke in my dream?
It maybe the joke was the dream-equivalent of a MacGuffin, a fuzzy, unimportant placeholder that wasn’t materialized into actual language, and that the point of the scene was to experience awkwardness, embarrassment, inferiority or exclusion that others had the upper-hand with in-group knowledge unavailable to the subject?

It would fit that what seems real in a dream isn’t necessarily real... that the idea of certainty came from the dream while waking up, but the actual content of that certainty likely never existed.