Well you just demonstrated that you don't understand the concept.
No, Elite Dangerous is not a sandbox. It's a theme-park open world game with no virtual player driven economy, credits have zero value outside of the game, there's no player driven market and no concept of supply and demands.
You don't need that for it to be a sandbox. It's just a word used to describe an open game where you can do whatever you want within the rules of it. There's no player driven market in Conan Exiles, but it's still a sandbox. You're given things to play with in whatever way you want and you do, that's it.