clearstream
(He, Him)
Well, strictly speaking I am saying that it is open to imagine mechanics diegetically, and I can recollect cases where we have done so at the table. Mostly, we've done so for humour, but we've also designed takes on specific mechanics like dice and inspiration that have made them diegetic. If anyone feels able to offer a robust definition that excludes my corner cases, I will gladly embrace it, but I don't think "game mechanics aren't allowed to be diegetic" works!@clearstream, I don't think your suggested usage of diegetic, as a property of mechanics, is very close to that suggested by other participants in this thread. I don't think they had in mind corner cases of the sort your usage captures.
My definition states what diegetic looks like when applied to roleplaying games rather than movies or books. It incorporates the fairly obvious point that unlike a movie or book, participants (via the ludic-duality) get a say over what their characters know. That in itself is useful. For example, BitD flashbacks have the glitch that a cost incurred in the past such as the ongoing consequences of trauma can't reasonably be retconned to already-played scenes that fell chronologically between that past moment and now. At least, not without replaying them, which is something I have never seen anyone talking about or doing. That continuity glitch is not explained in world, it's solved through participants simply acting as if their characters don't know it.
We're still discovering ways to adequately describe games, so there is value in getting our terms right even when we don't immediately see what that value will be. In this case I can certainly see glimmers of value right now, let alone what might be discovered in future.