I agree to the last, but not the first.
I think I am now ready to propose my own attempt at defining "diegetic" in context of mechanics:
For a mechanic to be "diegetic" it is required that all input to the mechanics is diegetic (in-fiction elements, knowable by people in the fiction) + a randomiser. This would for instance exclude dread, where there is a minor player skill element going into mechanics. Player or DM preferences are not allowed to enter it either. Safeguarding against a corrupt referee is generally not practically possible, but if a DM let their preferences affect the mechanics, the result is no longer "diegetic".
Okay, so how do we square that with--for example--the GM having complete preference control over the description of the outcome? Because some descriptions have significant impact, such that they condition future valid mechanics, while being, in D&D, completely and totally up to GM preference.
I'm really struggling to see how you could ensure that "
all input into the mechanics is diegetic" with D&D--non-diegetic inputs crop up frequently. Battle Masters, for example. Or even the Champion, simplest of the simple subclasses on the simplest of the simple classes: what is the diegetic input for the Defy Death subclass feature? You have advantage on Death saves. How can that be diegetic?
Furthermore for a mechanic to be "diegetic" it is required that all possible outcomes from the mechanics must be possible to understand trough a diegetic causality perspective from the inputs to the outputs. This excludes for instance pink elephants coming out of your nose as an outcome from a sneezing table in a setting supposed to simulate reality. In a simulation of a dream world this might be an acceptable outcome, as the dreamer might consider this kind of causality perfectly as expected. However, the output do not need to contain any more information than a simple binary "yes/no".
Okay....so....how do you
pick which one? Because it seems to me that that now means the GM can invent anything they want, whenever, whatever, so long as it has the tiniest scrap of plausibility, boom, instantly diegetic, always forever.
Again, I keep coming back to the notion of the failed Perception check as a critical breakdown point for this sort of assertion. The inputs seem diegetic; GM knows the party has just been exposed to something they might or might not notice, so she asks for Perception checks, essentially as a reactive defense; let us assume a DC of 15. The players roll. The high-Wisdom, Perception-proficient Druid rolls a natural 1: result 11. The 8 Wisdom, no-proficiency Sorcerer rolls a nat 20, result 19. The GM narrates this as the Druid being preoccupied with preparations for the day, while the air-headed Sorcerer was placidly looking out at the forest and spotted the enemy by chance.
Diegetic inputs, outcomes that are
possible to understand in the way you described, being entirely well-grounded and naturalistic, something that anyone could have foreseen as a plausible result. Yet we as players know that this is the GM
retroactively inventing the explanation, solely
because of the dice, not because of skill (since skill would have said the exact
antithesis of this event occurred). How can that qualify as diegetic? We're literally retconning the world in order to explain what happened!
These two conditions are sufficient for a mechanics to be "diegetic".
And some suggested relations to other relevant terms:
An activity guided only by diegetic mechanisms is a simulation.
A RPG need non-diegetic mechanisms, as otherwise it would hardly count as a "game".
A game with no diegetic mechanisms cannot be called "simulationistic".
Simulationistic play is play dominated by diegetic mechanisms.
That last one is gonna be a tough one to get through, because it makes such a porous boundary. What does it mean to be "dominated" by such? Does it have to be so nearly all mechanics that it's rare to ever see anything else? That would seem to "hardly count as a 'game'" by your own standard. Is it enough to be merely 50%+1? That would seem completely out of wack with how people talk about their expectations for such gaming. But leaving it open to individual interpretation just lands us right back in the same old problem of "so now any game is 'simulationistic' if the GM simply fiat declares enough of the mechanics to be so."