Without understanding all the technical ins and outs, it's interesting that it can run the same game made for Series X, so presumably 4K, and downscale it, but it can't run the X Enhanced versions and downscale or supersample those.
I still use a 1080p TV, as I still love the picture and gaming performance of my Sony 46W905, but whilst missing out on the HDR, RT, etc, bells and whistles, you can still see the graphical difference with X Enhanced and Optimised for X|S titles compared to their not enhanced/optimised versions when playing on the One X or Series X.