It's clear that you're frustrated, but I'm struggling a bit to work out what I'm missing.
Yes. And just to clarify - I think we are almost 100% aligned. The frustration was in that I felt like I had failed in conveying my message, and I couldn't determine why from your responses. This extraordinary post however gives me some solid holds to make a guess. I think I have identified some different assumptions we have without expressing them that could explain the confusion:
First is that I read Touvinen as proposing a formulation of sim that covered different ground than before. This as opposed to just clarifying the old concept. In particular I interpreted this as making some cases previously falling under the nar umbrella to be shifted into this new sim concept. This would invalidate earlier discussions related to the boundaries between nar and sim. As you are pointing to those sources in your analysis I assume now you are interpreting it as rather an attempt at clarifying the sim concept while preserving the same scope?
Unrelated to this we are also seemingly interpreting differently what the GM story hour say about the GM in particular. I guess we both share the understanding that this section describes how a spesific technique can support sim play? The way I understand that is that in this description sim play is presumed understood, and hence it do not in any way try to define sim. "The GM decides what play will be about, but the other players decide how they investigate that aboutness." I think is the crucial formulation we might be reading differently. You seem to read it as "In any situation where GM decides what play will be about, and the other players decide how they investigate that aboutness, we have a sim game"? I read it as "If you have a sim game, then a technique to consider is that the GM decides what play will be about, but the other players decide how they investigate that aboutness".
The important difference is that with my reading we can have a game where the GM decides what play will be about, and the other players decide how they investigate that aboutness, but that is incoherent! This is the situation I have tried to be exploring - so if you have implicitely assumed this could not be the case from your reading, it is no wonder if you have been confused.
I have also identified some other unexpressed assumptions I have been using that I am not certain you are sharing. So I will try to list them explicitely here:
1: Creative agenda is primarily defined on singular actions.
2: To talk about play as a whole to follow a creative agenda, all participants must follow that agenda (make actions with that agenda)
3: If the various players follow different agenda play is rather described as incoherent.
4 The GM is a participant in play per point 2 and 3.
(There are some subtelties related to hybrid play, and that 2 is of course never pure, but this is the simplified mental model I am working under when I have been making unnuanced claims like I have done in this reply chain)
So the spesific example I have been looking at is how the incoherent situation where a GM is joining with a narrativistic creative agenda, but the players are joining with a different agenda. That might on the surface look very much like "story time" - the coherent sim mode. The difference would just appear in cases where the agendas are put under stress. The standard failure mode here is that the GM start introducing elements because the GM themselves think it makes cool story, rather than for them to be interesting to engage with. NPCs that take over the narrative and outshines the PCs is possibly the best known of these.
And this is where the difference between homebrew and prepublished become critical. If you run Dragonlance, noone can suspect you for just pushing your own ego unless you happen to be Hickman. This is not so obvious with a homebrew setting. It could be that you honestly are curious to see what the players would do with it. But it could also be that you just want to show off the cool stuff you have made and gather feedback. One of these I consider "story time" in the sense described by Touvinen, the other I do not understand as "story time" but rather a ticking bomb of incoherence.