It's a marketing tactic. The manga is 23 years old and is "declining" in sales (relative to most other manga it still trumps 99% of them), they simply want to increase views/generate hype for Wano's 2nd half and the remaining story.
Mark my words, they don't mean "final arc" as in another arc like "Wano", they mean "final plotline" as in the Final War is gearing up. There will be a number of smaller arcs again to set this up. This is clearly evident of Marineford, where he spent 3 smaller arcs before he did the Paramount War. This is even more so evident in the last 400 chapters, where every single arc revolved around either Big Mom or Kaido, all culminating to some form of plot ending with them in Wano or beyond.
Now add that factor/logic and apply that to whatever the Final War will be, which will be vastly larger in scope than anything OP has done before. I'm not saying it will last another 400 chapters to tackle everything (though it might tbh), but it will certainly need a TON of setup before we can get to how everything will end. Yes, hopefully the pacing is improved and he creates less characters (that definitely won't happen), but it won't be short in the slightest.