Why would base game spirits disconfirm post pass characters? That's not a gray area, just a fan rule. I'll practice caution if someone gives a decent argument for it.
Exactly. The spirits we got with the base game were probably finalized before we even knew about Ultimate, and they probably planned the spirit events that happened during the Fighter’s Pass as they were putting the pass together- Those Astral Chain, Sword/Shield, and RE spirits were likely decided on before we even got Joker shown off.
I’m not sure when the second pass was greenlit, but regardless, if they wanted to use any Nintendo characters but arbitrarily decided they couldn’t use characters that were chosen to be spirits two years ago, they’d be shooting themselves in the foot. At this point they’d be limited to pretty much a Sword/Shield Pokémon that wasn’t already a spirit (brace yourselves for THAT storm when it hits), Porky, and Sylux (Neither of whom have any particular relevance or really strong fan support at the moment.)
Heck, even some third parties like Rayman or additional Sonic characters are walled off by this arbitrary fan rule.