Okay, let's argue like "adults", then.
You're asking Nintendo to lend one of their internal engines to a third party studio. Not even going into how many different layers of bureaucracy that would imply, since it is an internal middleware not meant for outside use and there'd be thousands of non disclosure agreements and overseeing to be had there; they'd basically have to learn how to use a completely new tool that they might as well not use ever again, for a single game, which also very likely has no documentation ever intended to be read by anyone outside of the company.
This, for a game that has been outsourced very likely because Nintendo doesn't feel they have the internal resources to develop in-house, in which keeping cost and dev time down is absolutely vital; and the most reasonable scenario is just letting the third party studio use whatever tools they have more experience with, in this case Unity.
And they need to do this so a turn based strategy game with stylized cartoon visuals uses "100% of the power of the console", because in this situation that makes absolute sense. Which is some hazy unexplainable concept that makes your argument sound, in the entire context of the situation, like a complete emotional preference based in no way in hard facts. Thus, yes, fanboyism.
I have no reason to doubt you have experience porting games to the Switch, but forgive me if I have serious doubts on the "knowing what you're talking about" part.