To me a sandbox means more than "you can do it in any order." Instead, characters having a meaningful in-game influence on the direction of their adventures within limits set by the DM which can be relatively narrow but also potentially global/planar.
Yes, you can use pre-published or other prepped material in sandbox and it still be a sandbox. I might say, when I first prep an area, "The Crypts of Istaris (which are detailed in the adventure of the same name in Dungeon #9) can be found outside the city of X in the kingdom of Y." If the PCs ever go there, I can drop some potential hooks, but I don't have to prep it or even more than do a skim to judge potential usefulness to place it. One they express their plans to go there, I begin to prep. But I do feel like I have wasted time if they don't, because I haven't.
Then again, today the PCs just flatly refused a hook for something I did take a lot of time prepping (even painted a mini!), but I am not mad. Their choice made sense. . . and I have to respect that. And that material will still be useful (if different with the passage of time) should the PCs end up going there later on.