A couple things are clear: 1) there are a lot of different definitions of "sandbox" in this thread, and 2) you are certain that yours is the only correct one.These details you fill in, do they follow the story the players suggested? (I get they might contain irony or some plot twists. That is not what I am asking.) Or do they discount any input the player had as they started?
I ask because once you "fill in the details" you are creating the story. It is not a sandbox. You can say, "But the players can choose to have their PCs turn around and leave." But the same is true for a "railroad" or "linear" or "hexcrawl."
If you say none of it is pregenerated, how is that sandbox? How does that represent sandboxes definition in any way shape or form? It doesn't. It is impromptu. You are making stuff up on the fly rather than having it prewritten (rehearsed).
The idea that the only real sandbox is one in which EVERYTHING is predefined is too silly to bother arguing with.