Psyonix is releasing a series of updates for Rocket League throughout the course of 2018. The team released an update roadmap which details what changes are coming. At the very bottom of that list, there’s a little blurb that mentions the Xbox One X:
Xbox One X – We are now targeting Xbox One X support for late 2018.
As you can see, Psyonix will be releasing the Xbox One X update later this year. As to exactly what time ‘late 2018’ refers to, it could be anywhere between September-December. That’s still quite a ways away, but at least we know it’s coming. No doubt this update will allow the game to run at full 4K; Rocket League isn’t an incredibly demanding game, so the Xbox One X should be able to pull this off no problem.