I'm sure people are going to disagree, and that's totally fine, but after speaking to a good amount of people on the topic, I don't know that the Rex Mii Fighter Costume Hard-Deconfirms him. While the wording can be interpreted many ways, and after viewing the Direct more than one time, I find that the wording could absolutely be interpreted as "If XC2 was out earlier, Rex definitely could have made the roster. Sorry, here's a consolation prize". I originally thought the entire section was about how all DLC was decided on before the roster was decided, and that may be the case. But my interpretation has changed.
Furthermore, the Rex costume is only available to fans who buy the DLC pass. It's not something that, as of right now, you can purchase separately. You aren't actually paying for the costume itself, so there's also the fact that even if Rex as an actual fighter came out as DLC down the line, it's not like a wasted investment.
Then, there's the fact that there's a Nia Mii Fighter costume in-game already which may not mean anything at all. It just seems weird that Nia would be base game, while a Rex costume isn't. Rex & Pyra are rather popular in Japan, so I'm not sure that the costume itself will actually completely satisfy the fans.
Another thing that may or may not matter, but a character who is only slightly older than Rex is Spring Man, and he was made into an Assist Trophy. While it's all fan-made conjecture, I personally believe that Assist Trophy status eliminates characters from the running for DLC character potential. Mii Fighter costumes, in my humble opinion (and I'm not just saying this because I want it to be true or something), don't disconfirm any character, considering we still have Link, Captain Falcon, Chrom, and even Skull Kid. Skull Kid isn't like the other 3, but I personally don't think it means he's totally out of the picture either. Basically, Rex and Pyra could have definitely been an Assist Trophy, but they aren't. The door remains open (potentially) for Rex to come later.
I don't know how likely it is for Rex to get into the game at this point. I hope he gets in...he's got the popularity, and at the very least isn't an Assist Trophy. I won't necessarily go on a huge campaign for it because I don't have the time or energy, and it may not mean anything in the first place. But I would place a Xenoblade newcomer, if there are going to be Nintendo characters as DLC, as having pretty high prospects.
In general, I wouldn't be so hasty in considering Mii Costumes or Spirits as disconfirmations. That would eliminate an extreme amount of options for sure, and there's already evidence to show that they don't disconfirm characters in the first place.