So what is a sandbox game anyway?

[Pleh]

The reason I say there is an illusion of meaningful choice is because as someone rightly pointed out, it is the DM that decides the outcome of every action.

The fact that the DM decides the outcome of every action makes a choice illusory IF and ONLY IF the DM has decided to provide the players with the same outcome regardless of their choice.

Just because the game is ficticious doesn't mean that elements of the game aren't real.

If the DM provides a set of choices for the players and the selection between these choices causes the DM to move the story along a path unique to that choice (and distinct from the consequences of the alternative choices), then the choice itself was real no matter how fictitious the scenario was.

Do not conflate an illusory scenario with an illusory choice. They are mutually exclusive properties.