Perhaps. But first and third party are not defined by what a person imagines might hypothetically occur at some undetermined point in the future. And console exclusivity does not define first/third party either - which was the point of my comment.
M+R is, by definition, a third party game.
Well, not quite. It was developed and published by Ubisoft, but you can't ignore that it is an IP collaboration, so it cannot be entirely third party. There are similar cases like Fire Emblem Warriors and Hyrule Warriors that were developed and published by Koei. M+R is published by Nintendo in Japan, FEW and HW are published by Nintendo in America and Europe.
You are looking at a combination of first and third party here and it's clear that none of the three games mentioned above can't go anywhere without Nintendo's consent.
If we evaluated a different, yet still similar case, what would you say whose game it is? The example I am talking about is Project X-Zone which is a collaboration between Namco, Sega and Capcom. Is it a Namco game? A Sega game? Or a Capcom game?