IGN:

Microsoft has confirmed that the Xbox Series S will not run Xbox One X Enhanced versions of backwards compatible games, and will instead run the Xbox One S versions of Xbox One and Xbox 360 titles with other beneficial features.

Microsoft‘s consoles names are a joke and this one paragraph hammers home how cumbersome they make it to explain what should be a simple feature. A commenter on Eurogamer summed it up best:

So the Xbox Series S can’t do the Xbox One X doing the Xbox but can do the Xbox One S doing the Xbox but the Xbox Series X can do the Xbox One X doing the Xbox?