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(He, Him)
In a videogame, a crate that character models can collide with is presumably diegetic. It's of course physically an arrangement of micro-switches. When imagining what characters know, it is open to imagine that they are aware of crates, but not bits; and it is open to imagine they are aware that crates are ultimately made up of bits. Just as we are aware of sub-atomic particles.Look, by any defensible definition of 'diegetic' there can be NO game mechanics who's operation by a player fits that bill. That's my conclusion. Rolling dice is not diegetic (aside from your peculiar case, perhaps), marking off hit points of damage is not diegetic, no forms of manipulation of a currency are diegetic, etc.
Defining "diegetic" is straightforward. What will be counted diegetic at a particular table is a matter of norms, including norms established by design. It's limited only by what can be imagined.
All that is required for a mechanic to be diegetic is for participants in the game to act as if it is.
But I have said this already, above.