Smooth Quartz is a block added to the creative inventory under Building Blocks sometime during the snapshots for 1.13. As you can see in the images below it looks identical to the quartz block until you look under it where it does not include a smooth surface like the normal quartz block does, instead it has the non smooth surface on all sides no longer making it smooth quartz. Smooth Sandstone however includes the smooth surface from under the cut sandstone (former smooth sandstone) on all sides, so perhaps the smooth quartz was meant to have the smooth surface from under the quartz block on all sides instead of the non smooth texture.