Couple things:
1#
They don't say "Fighter's Pass 2", they straight up say "Final Super Smash Bros Ultimate DLC character". If they said "Fighter's Pass 2", you'd have some wiggle room here, they didn't. They straight up said in this tweet it's the final DLC character, full stop.
2#
They announced the 2nd Fighter's Pass in Byleth's Presentation yes, but they had already announced "More DLC is in development!" back in the September 2019 Direct, which was a full 4-5 months heads up that more was coming. Sakurai's been saying since March of this year that FP2 is the end, this seals the deal. We've gone through 3 Directs where they could have announced "More DLC is in development!", and they didn't. They didn't in Feb, they didn't at E3, and they didn't today, and now they're saying this is the Final Smash Presentation.
I dunno, you tell me what's more likely. That this is all an elaborate ruse to hide more DLC is actually coming, or in 10~ days Sakurai will wave to the camera and say "It's finished." I know what option sounds more likely.