Ah, I was just mistaken about the origin then.
Fair enough.
And it doesn't need to be "mechanically unmediated" but the mechanics should be chosen to conform to the fiction rather than the other way around.
I tried to get this in my parantheses.
But certainly it must be about more than just 'referencing' fiction? Otherwise it is simply about how we phrase things.
I'm not sure what you mean by "'referencing' fiction", so don't know what the answer is.
Here's an example of non-fiction first resolution:
Each Burning Wheel character has an attribute called Reflex. It is derived from three stats (Agility, Speed and Perception). It will rarely be lower than 2, and if it gets to 8+ that's superlative. 3 or 4 is a typical number.
Reflex specifies how many actions a character can take in an exchange during Fight! Each exchange in Fight! consists of three volleys. Actions must be allocated to volleys such that, as near as possible (given there are no fractional actions), each volley has the same number of actions. Thus, the typical allocation of actions is 1 in each volley, but the possibility of a second in one or more volleys.
The allocation of actions to volleys - when they are taken, and what they are - is done at the start of each exchange, in secret. Thus we have blind declaration of actions, with uncertainty as to where they will land (unless someone has Reflex 3, 6 or 9 in which case we know there will be 1, 2 or 3 actions per volley). Part of the skill of scripting is to land your uncertain actions at a point where they are unopposed by the opponent. (Eg you Strike as your second action in the second volley, and with their Reflex 4 they have only one second action for the exchange and they've put that into their third volley - so your Strike will not be opposed by a Block or Avoid or Counterstrike.)
I can report from experience that scripting is tense, and the resulting play rather visceral. But it's not fiction first. It is nothing about the fiction that divides the back-and-forth of melee combat into a sequence of exchanges and volleys and actions; or that deems the spread of actions to be as flat as possible but with little peaks of uncertainty; or that deems one person (the one with Reflex 6) to act with a metronomic regularity that others don't display.
Those features of the action declaration framework are all external to the fiction. They're designed to support engaging gameplay.
A skill challenge does not have an action economy, does not use initiative (some early iterations toyed with this, but it was quickly abandoned - I can't remember, but maybe even the 4e DMG flags this as optional), begins all action with the fiction -
what is the situation - and ends all action with the fiction -
here is how the situation has changed. The fullest discussion of this is found in the DMG2, but it is also set out in the DMG.
In skill challenges the situation is resolved after fixed number of fails or passes because that's the mechanic, and we need to weave fiction to conform to that. The challenge doesn't end with five successes because that makes sense in the fiction, instead it ends with five success because that's the rules, and we (hopefully) can come up with fiction to explain why those five checks were made and why those specific deeds would resolve the situation.
In Apocalypse World, if I succeed on my attempt to Seize something By Force then we have to weave fiction to conform to that outcome. We don't independently consult the fiction to work out whether or not it "makes sense" that I have seized the thing by force.
But it would be an odd result if one of the games best known for "beginning and ending with the fiction" turned out not to count as "fiction first".
This is why, as I've already posted, you seem to be using the phrase in some fashion different from the way I have generally understood it.