Honestly, if any Xenoblade character didn't get unfairly pushed aside, that would not be Rex. Again, he was too late for the base game, and no, Byleth doesn't counts since they're DLC and their game released post Ultimate so there was an actual planning window.
Instead the one who really got nothing was Elma. Or you could argue the avatar, but either of the two.
X was already out for years then and thus was the latest Xenoblade at the moment. Then again, I guess unlike Splatoon, not having anything on Switch didn't give them as much of a chance.