This brings something to mind that might be a bit of a tangent:
What is the difference between the above, and the situation where the player asks, "Hey, do I see a market stall near the wall, preferably where it is at a low point so maybe we can get over the wall that way?" and the GM, who hadn't thought of it before, decides "Sure, there's a cabbage stall next to a spot where the wall dips low."?
Is there a difference? Does knowing for certain they are inventing a detail impact the players? Is it not "player authored" if the Gm just goes with whatever the player says without acknowledging that the player made it up?
There's a sort of table etiquette difference - do you ask if it exists, or state its existence and wait for the GM to shoot it down? It is a table-agreement sort of thing.
But otherwise, I see no meaningful difference, and I would generally accept the player's suggestion that the stall exists, so long as taking advantage of that stall still requires a roll.