It's a weird one. The platform is definitely recognised by the site but it's held as a sort of sub-platform to the predecessor console by default, for some reason, so if you press the plus icon beside the platforms on a game page, it may reveal additional platforms if the mods have specified that submissions are valid on them. So don't worry, the platforms are definitely valid for speedrunning
The best bet is just to ask the game moderators if the Xbox Series X/S is accepted for submissions, in all my experiences with modern console games thus far that's never been a problem, especially when an Xbox One game is ported over, but there might actually be platform differences which warrant a different category for it. If there isn't one and Xbox One is listed, and it's available on the X/S, I'd just poke the game moderator(s) to check. It'll depend game to game :)
