Given how much custom work goes into some of these, it's really not so much a unique emulator as it is a unique classic game remake engine. The emulation is more of a front, to use the "emulators are legal" defense.
Like yeah, it can just generate plain voxel art from the sprites, but the ability to make entire custom designs to replace that art, add depth and whatnot, is the main thing, and in that regard it's really more of a remake engine. It hardly even needs to be voxels at that point, it could just swap in custom 3D models when it detects specific sprites.