Rex's outfit is specifically being offered as a bonus for buying the Fighter Pass, but why would they include him instead of any other Mii outfit? We don't know if the outfit was made for the Fighter Pass specifically or if it was made alongside the base game's Mii outfits. It's possible that it was created for the base game, and was made an addition to the Fighter Pass when Sakurai decided to make DLC. If it was made independently of the Fighter Pass, it would answer the question "Why would they bother making a Rex outfit if they were already planning on making him playable?" if it wasn't made with DLC in mind.
We don't know how exactly "we decided which fighters to include when we started planning" applies to Rex. Obviously he was too late for the base game, but was he too late for DLC planning too? That doesn't make sense unless the DLC characters were decided on alongside the base roster, which doesn't make much sense either because Sakurai doesn't think about DLC until the main game is done. We don't even know if the five DLC characters have been decided on yet. If the statement only applies to the base game, then it has nothing to do with Rex's DLC chances.
There's the idea that Rex has a Mii outfit because Sakurai isn't making him playable, but why wouldn't he? His game did well critically and commercially, he's a popular request, Sakurai likes his game, and he has the potential for a unique moveset. So why wouldn't Sakurai want to include him?
There's also the matter of why Sakurai took the time to make this statement to begin with. The majority of other deconfirmed characters are lucky to get a passing mention, much less a segment of a Direct dedicated to them. All of the Xenoblade content mentioned here is from 2 specifically, so this whole statement reads as Sakurai apologizing for being unable to include more Xenoblade 2 content in the base game. Rex is one of the most popular requests in Japan and Sakurai is a fan of XC2, so it makes sense why Sakurai went out of way to explain the absence of content for this game in particular. It makes me think that Rex would have been in the base game if timing was more convenient.
So basically, Rex's chances depend on the answers to these questions:
- Was Rex's Mii outfit made specifically for the Fighter Pass?
- Has Sakurai decided on the five DLC characters yet?
If the answer to these questions is no, then the Mii outfit should have no bearing on Rex's chances. If anything, this
Sakurai bias rare acknowledgement should make Rex more likely as DLC, not less.