I didn’t see my answer as contradicting Schooner. I see it as us both saying that a BW test moves the plot forward: if it succeeds, the player has advanced to the next dramatic step of their dramatic goals; if it fails, the player has a new dramatic challenge to overcome to achieve their dramatic goals.
Stripping away the subtlety, this could be expressed as Drama!.
Both having someone happen to crest a dune and having the player travel from the middle of the desert to Austria require suspension of belief: the first for the coincidence of the meeting, the second for the coincidence of avoiding all the vast risks without even needing to test. Where they might differ is in Ob, Forks, Consequences.
So, if a player wants someone to crest the dune because that’s what they think drives their plot forward, perhaps the Ob is high and the consequences are being captured by hostile tribespersons; whereas if they just say they want to find a person, the GM can choose where it happens depending on other things and perhaps have a lower Ob and less drastic consequence.
I see the “justification” of the person being available as a combination of Circlee qualities, Consequence, and “does it matter?”: what might go wrong defines at least one edge of what might go right, and for the rest does it matter why they are there?
For example, the player decides they want a hot-air balloonist in the middle of the desert, the GM could set the consequence of failure as “they’ve crashed and need help to repair the balloon”; moving forward to the point of having a working balloon somehow, the player wanted the balloonist to balloon not for backstory, so potentially the next step is “the balloonist floats you easily across the enemy tribe’s range and lands you on a plateau saving you weeks of dangerous travel” and the question of why their was a balloonist there isn’t actually relevant to the drama. If the players want that balloonist to be acting from particular reasons, then that’s likely to be part of the Ob of the Circles test, so if the backstory does matter, the players define how it matters.
Or to put it another way: if a game system can include dwarves using seduction to rob a dragon, it can include someone happening to ride over a dune at just the right moment.