The Series S doesn’t support the enhanced games on Xbox one X . You get bog standard versions
Well aware of that, hence "in some cases".
Series S will be running the One S versions of games, but at their maximum resolution (where dynamic resolution scaling was used), with improved texture filtering, auto HDR, and (in select cases) at double the frame rate. Auto HDR and frame rate doubling are features not seen on One X. And frame rate doubling aside, all of these enhancements are automatic, applying to all Xbox One titles. So there will be plenty of games which saw no One X enhancements, and will therefore look and run better on Series S.
Xbox and Xbox 360 games will run at 3x and 2x native resolution, respectively. So 480p Xbox titles, and 720p 360 games, will run at 1440p. Auto HDR will also be applied to these games. Again, these are features not seen on One X.
On top of that, there will be games like Gears 5, Forza Horizon 4, Sea of Thieves and The Witcher 3, which are getting next gen enhanced versions. These versions are going to look better than their One X counterparts.
To pick on two games as examples, Battlefield V will look better on One X than on Series S due to the One X enhancements. But Battlefield 4 is going to look better on Series S than on One X as there is no "One X Enhanced" version.