But if they give me the source under the MIT license then I am free to redistribute it.
So while they would be in their right to make a proprietary port it seems like they wouldn't be able to legally release it under MIT. (Or at least not the whole thing, as code that touches Nintendo's API would need more restrictions)
My understanding: it is possible to license under a different license any additions or modifications they made. So the original code can remain MIT, but their changes (API integrations, etc) will be under a proprietary license (note: this would not be possible with GPL for example, but MIT is not like that).
So while they would be in their right to make a proprietary port it seems like they wouldn't be able to legally release it under MIT. (Or at least not the whole thing, as code that touches Nintendo's API would need more restrictions)