S'mon and
pemerton already gave some good responses, but I'll add a few specifics:
In otherwords 'simulationism' through 'stripping out the simulationist elements of the game system.'
It's unfortunate that "simulationist" has acquired a "common" meaning that I find ambiguous, loose and not very useful. As far as I can make out, it is usually taken to mean "system heavy" or "sticks to 'simulating' elements from the "real" world", or some combination of both of these. For the main streams of roleplaying - fantasy (where sticking to the "real" world is just limiting) and sci-fi (where the idea of
not simulating the "real" world to some degree is anathema) - this usage seems to me to be redundant and unhelpful.
The Forge meaning is much more unambiguous, centred on actual play aims and interesting to me personally as an "ambition of play".
Is it just me, or did that essay just find a way to define simulationism as simply a complex term for make-believe, stripping out the game system elements itself that attempt to aid in simulation, as opposed to the common definition of a game system being used to simulate elements of a world in opposition to arbitrarianism.
It defines Sim as 'make believe'
for its own sake. All roleplaying involves make believe to some degree to make it work; for those times when you are focussing on a Sim agenda, exploring that make believe together is the main purpose of the activity.
Taken from this angle, Sim does not need extensive rules, at all. It may take a realm of 'fluff' that describes a setting and base play on "rules" and guidelines that are developed during play based on that fluff. Planescape seems very suited to this approach, to me. The danger of "arbitrarianism" comes in if (1) the game is operated with an "all powerful" GM and (2) that GM begins to see/treat the game as a competitive excercise, either spontaneously or due to some "gamist" behaviour on the part of the players. A good way to counteract this, in my view, is giving a substantial part of the traditional "GM's power" to the players or the group as a whole (something you should never do in a gamist game).
When you live in a realm where belief that everything is chaos and disorder actually gives you the ability to point at someone and remove their ability to speak coherently, or where your belief that the universe is best when put under your benevolent dictatorship gives you the ability to change minds to befriend you and follow your word... that's what -clubs- means. The Fated have a greater propensity for self-sufficiency, the Sensates have greater immunity to negative experiences... in the planes, what you believe is both a source of power and a battleground.
Yes, I realise all that - but that isn't really "philosophy", as such, is it? It's a set of hypotheses concering a setting where belief has real power - hypotheses that might, themselves, be philosophised about, but that are used/wielded by believers, rather than by philosophers.
Interestingly, I think this would be a good use of themes in 4th edition, to reflect the abilities granted by faction.
I think you could represent the factions using themes, yes, but I think that all PS would be in 4E is a pretty backdrop for a challenge-based or theme-based game. I don't see all that much point in using the setting that way, but YMMV and it should certainly be possible.