The disagreement is in the inherent meaning contained within game mechanics.
In the relevant example, regarding a player looking to bypass an enemy's armor, Pemerton would have the quality of that armor be determined by the result of a Perception check made on behalf of the PC looking for it; I view this as improper procedure - not in keeping with the purpose of the rules in an RPG - because the quality of an object must exist independently from observations made of that object
Upthread we agreed that the gameworld is causally independent of the real world.
But your description here of "proper procedure" seems to violate that independence, because you are saying that the meaning of a Perception check (a real world event, involving rolling dice and adding up numbers and perhaps looking at DC charts, etc) is somehow embedded in or causally connected to an ingame event, concerning something that "already exists" ie something in the gameworld.
I take it as obvious that, within the gameworld, armour either has a chink or it doesn't. And, within the gameworld, when a character looks hard to try to discern such a chink s/he either sees an existent check, fails to see an existent chink, or sees no chink because there is no chink to see.
But that fact about the gameworld doesn't settle any real-world question of
who gets to author the gameworld, and
how is that determined. In AD&D, and at least as far as I interpret the system in d20 D&D also, a
player's atttack roll can determine whether or not an NPC parries successfully, at least to the extent that
if the attack roll is a hit, then it follows that there was no successful parry (contrast with, say, Runequest or Burning Wheel, where the player of the defending character is entitled to a parry roll to prevent the successful attack roll actually amounting to a hit).
I think that the player authoring of defender parries in AD&D and d20 is obviously a fairly weak sort of non-simulationist authorship. The example I gave from BW, of rolling Perception to make it the case that a character spots a chink in the defender's armour, is more elaborate than rolling an attack to make it the case that the defender's parry fails. But I content that they are processes on the same spectrum. And that D&D has always had combat processes at various points on this spectrum (it was precisely in reaction against these features of D&D that games like RQ, and a few years later Rolemaster, were designed).
Pemerton seems to be of the opinion that, since this is all just make-believe anyway, there's no difference between discovering some truth about the fictional reality and defining a truth about that fictional reality.
I'm of the opinion that, because it's all make believe, someone has to write it. And because it's a game, there have to be procedures (rules and guidelines) to regulate that authorship.
A player might discover the truth about something the GM wrote. Or a player might declare that s/he wishes something to be true in the fiction, and then roll dice to see if it's true. And that roll might be modified by choices that the player made in building his/her PC. In AD&D, choosing to play a fighter rather than a thief makes it more likely that your opponents will be ineffective at parrying; that's a modest example of the same phenomenon.
Which goes against the premise of any sort of Simulationist/ immersive RPG, where the whole point is that we're pretending this stuff does exist in some objective configuration before we discover it, and the PCs aren't just characters in some story. To play the game is to buy into that illusion, and that's presumably what everyone has signed up for.
This differs from a Narrativist/ storytelling RPG, where the point is just to tell a story
You are misdescribing the point of narrativist RPGing. If the point was to tell a story, we wouldn't use RPG devices like assigning each player a character as a distinctive vehicle for engaging with the game and the shared fiction.
The point of narrativist RPGing - at the risk of being oversimplistic - is
story now but
within the framework of RPGing. That is, that play will generate, in the moment of play, a
story in which these characters are
protagonists. Part of the function of the game rules is to bring this about
without the participants actually having to work at creating a story. Good narrativist RPG mechanics will facilitate the emergence of a story organically, with the individual participants not having to do anything beyond their distinctive jobs as GMs and players.
For instance, if a player could just declare at will that his/her PC finds a chink in the enemy's armour, we wouldn't get a story (in any non-degenerate sense) because there would be no failure, no rising action etc.
Because narrativist RPGing is RPGing, it does not differ from simulationist RPGing in respect of the convention that you mention: namely, that everyone at the table is pretending the fiction is real; and it is wrong to play the PCs simply as if they were fictional characters (rather, the player's job is to inhabit and advocate for the PC). Where it does differ from simulationist play is in the constraints upon and rationale of authorship.
the distance between Sim and Narrative seems the hardest to bridge.
In playstyle, perhaps. Not in mechanical design. I've run narrativist RM, though I don't think that's what the original designers of that system had in mind. I'm sure there are plenty of 4e players who have run the game essentially as sim (though not necessarily tight process-sim), with the players exploring/discovering the GM's pre-authored world and plotline.
An early example of sim-to-narrativist drift would be relationships of various sorts in Champions - these are introduced to model superheroic genre conventions, but then the participants discover that they also serve as player flags to the GM, which permit the GM to author backstory and frame situations in a way that speaks to the PC as protagonist and not simply gameworld inhabitant. Before you know it, its Story Now!
I've had the same thing happen GMing an all-thief party in AD&D (thieves, like paladins, monks and perhaps druids, are AD&D classes with strong thematic hooks built in), and also in Oriental Adventures - in the latter case, relationship mechanics (ancestry, senseis, etc) designed to serve a sim function in fact end up serving a flag-flying function, much as I described for Champions.