doesn't answer your question but here are a few notes
the xbox one x was the "4K version" of the one s - so, was the one s ever 4K?
displaying a picture is not the same as playing a video on Kodi, a picture certainly may be scaled to the GUI but that is not the case with a Video so the OP on that post incorrectly tested 4K 'video'
xbox one series was discontinued 2 years ago, if it hasn't received proper updates then the new version of kodi may not even run on it
the post is regarding kodi 19.3 when the current version is 21, they are not going to be the same 'at all' - lots of changes in 2 years
your post indicates you would like an opinion on this as well, my opinion would be to not buy a 7 year old device
'cheaper' alternatives that play 4K, pretty much anything that isn't total crap these days will play 4K, Chromecast 4K, Firestick Max, FireTV Cube, Homatics, Dune and so on - see this post
https://forum.kodi.tv/showthread.php?tid=376035