My working theory is that Nintendo's partner showcase does have some Xbox titles in it and has been planned for today (maybe?) the entire time.
But then there were meltdowns from Xbox influencers thinking Xbox is leaving the console business because it was said their 1st party titles are going multiplatforms, Microsoft/XBox had to pivot.
Nintendo got wind of this XBox Podcast event, opted to reschedule the Partner Showcase, which would've not been a big deal (not from their POV, but is big deal here and Twitter, etc apparently), because part of those meltdowns were related to Xbox 1st party going multiplatform specifically, so it's a bit too on nose, they thought why not just reschedule?
It's not the first time we have gotten 1st party Xbox titles either: Cuphead and Ori and the Blind Forest, for example. It's weird that there are meltdowns now all of sudden, whereas there were none before (or were there meltdowns in the past too over those?)
I agree, I think Nintendo felt like it was no big deal to reschedule due to the recency of those meltdowns, especially if it's just a partner showcase.