Edwards and Tuovinen don't offer identical characterisations: sincerity is not the same thing as elevated appreciation and understanding in virtue of intensely detailed perspectives. What they have in common is that they exclude certain sorts of ways of establishing the shared fiction:
"Simulationist" RPGing, as described by these commentators, precludes altering the fiction on the basis of, or as a direct response to, our appreciation or engagement with it. This is not easy to state, and not easy to achieve. In a RPG, the fiction has to change. Necessarily, those changes occur because the participants engage with it - they describe fictional events, fictional causes, fictional effects. "Simulationism" is about trying to hold the fiction constant on its own terms while the RPGing happens.
(Emphasis mine.) You make an important observation here. There ought to be a third characteristic speaking to how the fiction can be altered. To my reading Sorensen aims to solve that via insistence on players engaging only diegetically. But before continuing, I want to make clear one thing that others might disagree with...
Often (not always - consider some approaches to Tuovinen's "dollhouse play" or "substantial exploration") it is the job of the GM to hold the fiction "constant on its own terms", meaning that the GM is not really getting to engage in the sincerity and the elevated appreciation. They are labouring away, fully conscious of their authorial role, so that the players can enjoy the pleasure of simulationist play. I think it's actually rather common to see GMing described in this way, often using metaphors of "behind the screen" or "behind the curtain".
I do not take "simulationism" to necessitate a GM. I don't see anything at all about simulationism that requires it. It's a good, workable option with a lot of benefits, but it's not the only option. If others think it has to have a GM, then they should just bear that in mind in how they understand the third characteristic.
iii) play alters the imagined world diegetically
That frankly steals directly from Sorensen, only freed from assumptions about a GM. Can players be relied on to give the imagined world precedence, address it as independently real, and not alter it during play other than diegetically? I don't see anything inherent to what it is to be a player that prevents it.
Like the others, this characteristic is also intended to apply to how the game is designed. It leverages my contention that game mechanics can be diegetic, but constrains designers to not offer non-diegetic mechanics. (One ought then to say how to separate them, but I will leave that for another post.)
To summarize how things are evolving, back in
#13,237 I glossed
Sorensen as saying
Setting precedes abstraction precedes play; so that setting indeed serves as reference.
Abstractions exist only to make the details and dynamics of setting available to play; knowing that this will be incomplete.
At every moment of play, players contribute to the fiction only that which is faithful to setting; where "GM is a player."
Whether that's right can best be
judged by reading his words. Here I'm proposing three characteristics that through emphasis upon them identify a family of things a game can do as "simulationist"
i) the imagined world has precedence
ii) the imagined world is addressed as if it were independently real
iii) play alters the imagined world diegetically
These are intended to bind on designers as well as players. They're not all or nothing. They're expected to be productive of doing something that can be recognized as simulation, without guaranteeing it. They're silent as to your motives for wanting to do that. As I write this I feel I'm mirroring what Sorensen has said, albeit removing the reliance on GM.