If you want to isolate just an object rendered in sketch onto a transparent background, you could just cut or erase out the opaque background or render over flat white with no ground and again, multiply. There's several solutions I think. Save out the target layer, after isolating and ridding the BG, deleting the rest, as a png without flattening (which will flatten to an opaque background). You'll end up with your sketch rendered focus on transparent background file.