If y'all allow me to go a little crackpot for a minute...
That leaked Sakurai column said there are no plans of new characters past the 6 of FP2. They haven't planned anything yet. But the thing is, right now we don't even know so little as who fighter 6 is, so we have a loooong way til the end of this pass. Going by FP1's rate, that'd be just over a year of new characters, even more so if you believe the December 2021 deadline and that they're stretching out reveals and releases that much. Either way, that's just enough time for planning, negotiations and all that jazz to take place. So yes, Sakurai is telling the truth and there's no plans for more content right now, but over the course of FP2's lifespan, they could very well start to take shape.
... do I actually believe that's the case though? I don't know. I'm always up for new characters and the speculation surrounding them and they always deliver, but I also see that the Smash team and especially Sakurai have worked their ass off several times over and FP2 will probably be a damn good ending for Smash Ultimate, if they really do set out for it.
So tl;dr I'll still be pushing for FP3 or at least some individual characters after our current pass a little, but for now I'm just happy we're getting more fighters at all.