I don't think the second -- two individuals correctly identifying something as both diegetic and not -- can be ruled out for TTRPG. Roughly
- Given X is diegetic if players can pretend their characters know that X
- And to pretend in play requires players to willingly entertain beliefs... to voluntarily imagine X, in other words
- Where the voluntary imagining is done in accord with principles and rules put in force for oneself, so that things in discord with those principles and rules will not be imagined... not entertained or pretended to be true
- And there are differences between players in willingness to put some given principles and rules in force for themselves
- So that there are differences in their willingness to entertain certain beliefs... those players may differ in their willingness to imagine X, in other words
- It is entirely possible that some player A will have a willingness to pretend their characters know X, so that X is diegetic for player A, whilst player B does not share that willingness, so that X is not diegetic for player B
Such differences in willingness as I describe have been abundantly testified to in this very thread! Importantly, I do not think it does any harm to the concept of what is diegetic in TTRPG that there should be such differences. It seems inevitable for a form of narrative whose medium is the imagination.
Unlike the shared experience of watching a film and hearing music in the auditorium, some players could refuse to imagine that music. It can scarcely be described as diegetic -- it would not even exist! -- in the world they are
imagining.