Bedrockgames
I post in the voice of Christopher Walken
A sandbox can very well feature a menu of adventures. A valid sandbox can be a town, a bit of wilderness, and a bunch of themed dungeons. This is exactly a menu of adventures. There's no one true sandbox. Granted, you're making a reasonable point if you're contesting just the fact that a sandbox need not be a menu of adventures, but you aren't doing any favors by returning a "must" with a different "must."
The point is, Sandboxes are not just a menu of adventures to select from. And the reason is the living world/world in motion component. The players actions, particularly with NPCs and factions, but with other elements of the setting, generate adventure content that the GM never considered, thought of, or would have come up with. This is very important. And it is something that doesn't happen in a lot of games. The players are constrained by being locked inside their character. They don't have power over the setting. But like a person in real life, they are free to go about and do what they want within those constraints (and those are constraints but I think constraints that are meant to approximate real world ones, a pretty reasonable boundaries for agency)