Of course, I understand. My main point being there would no doubt be internal discussion for the inclusion of Banjo & Kazooie rather than Phil himself outright declaring it so.
There'd definitely be an internal discussion, but I see no reason why it'd be a difficult one. Microsoft is generally very relaxed with their properties nowadays, with their support for Minecraft on all platforms (including having content from Halo and, yes, Banjo Kazooie in it on all platforms, including the Switch), putting their exclusives on both Xbox and PC, and spearheading crossplay efforts. There's no evidence that Microsoft would be difficult regarding any of their IP's in Smash, but plenty of direct and indirect evidence that they'd be very open to it, which is why I'm very confused why people keep thinking it'll be a problem.
Even if everyone at Microsoft went full-throttle on getting Banjo in Smash, there's still no guarantee he'd get in. Nintendo is the one who ultimately pulls the strings. Regardless of how much support Banjo gets from Smash fans, it's up to Nintendo to make that call, not Microsoft.
Indeed, it's up to Sakurai/Nintendo to request the character in the end. That's how it's always been, and that's really the only thing holding Banjo back right now. But the door is wide open, so if Sakurai makes that decision, it's as good as done.
In a hypothetical scenario where Nintendo contacted Microsoft about B&K's Smash Bros. representation, and Nadella was okay with it, then the only hurdle left is whether Nadella agrees with B&K, or if he would say, "Well, why don't we suggest a franchise that would make us more money?" He's the only one Spencer has left to answer to.
That said, he's shown an enormous amount of faith in Spencer already by promoting Xbox to equal footing with Windows and allowing a total upheaval of the brand in recent years, with Play Anywhere, the One X, Game Pass and offering two different generations of backwards compatibility for free. This isn't business as usual for Microsoft when compared to how they've handled things since the X360. What they would or would not do regarding Smash is suddenly a total mystery.
Furthermore, Nadella isn't a very stingy guy. He's the one who really pushed for Microsoft Office to be on mobile devices, including their competitors, like iOS and Android. He's also been committed to maintaining Minecraft support on all platforms, rather than lassoing it to just Xbox or something. I doubt he'd micromanage any Smash content.