You are mistaken, a sandbox MMO, if you insist on comparing only within the very exact same genre, should be compared to something like EvE.
That description simply means that it is an mmorpg where you have no guiding main quest and storyline that you’re forced into following, and is used to differentiate those games from the other type of mmorpg, like WoW and such, that are called ‘themepark mmos’, where you’re locked into following a series of quests and a story.
Trove is a fair comparison too, but this is just about the least grindy one out there, if we’re comparing only sandbox mmos, Trove included. Not to mention that the cash shop is entirely optional, unlike, again, Trove, where you have to pay to unlock game features such as being able to use a particular class.
It has as much in common with Minecraft as with Cube World, namely the characteristic voxel look with worlds made primarily of cubes. But you’re not comparing visuals, you’re comparing game mechanics, and so in much the same way that mechanically you cannot compare Cube World with Minecraft, even tho they’re both single player games with voxel looks, you can’t really compare the mechanics of single player games and an the mechanics of online-only mmos just because both use cubes to build worlds.
They will be too different, by necessity, in terms of game mechanics, for the comparison to be meaningful at all.
Edit: Here’s an article that defines both types of mmos: