Considering past Mii Fighter announcements such as the ones from 4, of characters who were not playable nor ever playable to begin with, it's a bit precarious to treat them like beacons toward things to come. The Inklings and K.Rool in particular were given costumes for Smash 4 DLC. At the time, it was fair to assume that both the costumes and the characters could be playable regardless as the costumes are just cosmetic and other playable characters had their own costumes already. However, that didn't turn out to be the case. If a non-playable character got a costume before their debut, that character is out for good.
Fast foward to Ultimate and we finally get the Inklings and K.Rool in the base roster, but it still took a new game to make it happen. Now put yourself in Sakurai's shoes for a moment and consider the circumstances behind Rex: A costume of him was made DLC for a combination of reasons, first being that he was too late to be made playable either for the base roster or DLC, and the second being that Nintendo did not have faith in Xenoblade Chronicles 2 succeeding. Therefore, the drive (no pun intended) to promote the game even further through Rex might not be there. Second reasoning is the worst case scenario here.
However, there is a silver lining to that very scenario. Assuming our Fighter's Pack does a phenomenal job at sales, this might encourage Nintendo to revisit the idea to pump out
even more DLC. At this point, we can speculate the possibility of Assist Trophy upgrades as well as Rex. We would be so far ahead in the development cycle post-release that he would gain his chance at being playable. They would know that Ultimate is the final (good) Smash game and the ultimate
(okay...pun definitely intended that time) last chance any character like Rex has at being playable before becoming irrelevant in their eyes. Again, there is
no reason to make his Mii Fighter outfit if he's intended to be playable, or even
considered to be playable because it would be redundant to do so.
At least in this Fighter's Pack. Time will tell what will happen to Rex and the others afterward.