ON the contrary that wouldn't make sense.
I'm afraid this video says otherwise in the evidence department. Note that the Mii costume of Rex was given as a bonus for those who purchase the DLC season pass, which means it would come out alongside them.
Now
MajestyIndigo
why would they include a mii costume with the actual Rex if he and Pyra were a part of the Season 1 pass.
Also, unless Nintendo are trying to pull the wool over our eyes, they pretty much said that they know what those dlc picks are, by not showing us what they were yet, revealed. And let's not forget, fighters often take almost a year to develop, and if this is being released across the course of a year, then yeah, it would need to have been pre-planned.
And back to the main point, Rex is a consolation prize along with the dlc season pass. That means that he couldn't be fitted into the DLC plan since he and Pyra would have been an obvious selection for DLC.
It takes more stretching to assume that Sakurai was talking about the base roster than the DLC, since it's all the more illogical. Otherwise, why not announce something like a ballot or urging fans to campaign their characters to try and have a say in garnering their interest for these first 5?