See, I don't want to say, definitively, that that's what is going on. It seems like it. Why start with ARMS first, especially if you know you have something bigger. And it's clear Nintendo is well aware of the timing. Don't think it's any surprise they announced Hero and Banjo at E3 and gave Byleth a unique presentation outside of any Direct. And I don't you can argue delays and "switching around" as, despite COVID-19, they'd still work in order of what they had.
My reservation is that who's to say that given FP2 they wanted to add ARMS (which they initially couldn't) and now they can, but everything else is the same? I can see that as a possibility too. So while I think you're right, that ARMS sets an exception of what's to come, I worry this is just a rationalization on my part (as I desperately want more Nintendo characters). That said, I would be happy if we get 3 more Nintendo characters and this is the reverse of last time.
I agree on Sword and Shield, but don't think you can say that about Rex. They probably wont add much from the Remaster as XC1 already has a lot of content in Smash. Also, from Terry it's clear they don't care about how dated or not the character is. In Rex's case, it was clear they would have added him and the way they presented the costume kind of implies that (they almost flat out apologize for not adding him). I think that given another run of DLC, they'll consider it at least.