The map data is voxel based, but it's transformed to polygons when rendering.

In a 2D game, the drawing primitives are triangles, the map is made of square tiles, and the detail is in the texels.

In Minecraft, the drawing primitives are triangles, the map is made of cubic tiles, and the detail is in the texels.

I've always disliked the use of the word voxel when talking about Minecraft, because I think it applies too loosely. They are mostly map tiles.