Contributing to the shared fiction is not sufficient to call something an rpg, else pass the conch would be an rpg.
I didn't say it is. But it is part of playing a RPG.
An RPG is about contributing to the shared fiction via a player controlled character. Just so it’s clear I believe all your games have tons of this, but I believe these games have typically been structured in such a way that what actually happens in them is hard to talk about. There’s characters, in fiction acts, authoring, etc. But these games tie out of character authoring of things the character cannot control to player action declarations for their characters. I described this as overloading the action declarations earlier in the thread. This overloading doesn’t actually solve anything, it just makes it really difficult to discuss what’s really happening.
There is not the least difficulty in saying how MHRP works:
*The PC is subject to a Lost in the Dungeon complication, rated at d12.
*The GM frames a scene, that includes a Strange Runes scene distinction.
*The player, as their PC, has the idea that the runes might reveal a way out, or at least help work out where in the dungeon the PC is.
*The player therefore has their PC read the runes, with the purpose of the action being to reduce or eliminate the complication.
*The respective dice pools - one for the PC, one for the Doom Pool - are put together, and rolled, and totals and effect dice established. The player's pool includes Solitary Traveller and Cunning Expert - these are the features of the PC that make them apt to be able to reason about, and potentially read, strange runes. The GM's pool includes the complication.
*The player succeeds, on this occasion as best I recall with a d12 effect, and so the complication is completely eliminated: in the fiction, the PC has read the runes and realises that they reveal a way out of the dungeon.
The action declaration is straightforward. Its resolution is straightforward. What is happening in the fiction - that is, the character is reading the runes and thereby learning a way out of the dungeon - is quite clear. The only reason it is perturbing anyone is because
some prior GM authorship of the meaning of the runes was not part of the action resolution process.
The complaint here is the player is manipulating the game world outside their characters.
But the player is not "manipulating" anything but dice. They are contributing to the authorship of the setting - in this case, the runes. But that is not "manipulating" anything. When JRRT wrote LotR, he wasn't "manipulating" Middle Earth. He was creating it.
The mode of creation in RPG play is different. But it is still creation, or authorship.
If you go reread the original runes example just posted this looks to be exactly what happened there. The player was lost and wanted the runes to provide the exit, ie solve the characters problem.
But the player did not "dictate reality". They contributed to the authorship of fiction.
You don't object to a player solving the problem of their PC being attacked by an Orc by declaring actions and rolling dice. Nothing different happened in the runes case. Your objection is simply that the action resolution did not include or make any reference to prior authorship by the GM.