Does it literally say "arc"? Or does it say "Saga" (not sure if Eten can clarify for us)?
If its "arc", I believe Oda has referred to the entire Yonko plotline as an "arc", not a saga, so that means the last 300-400 chapters could be deemed an arc to take down the 2 Yonko., meaning that whatever is next could just be a conglomeration of arcs making up the "Final War".
If its "saga", then its essentially the above but bigger. Oda has only used the word Saga twice: The first for the Sea of Survival: Super Rookies Saga and for The Final Sea:The New World Saga. If its the last "Saga", then that is Oda effectively telling us that there is essentially another "3rd" of One Piece remaining in terms of Saga's indicating bigger plotwork at play here.
Regardless, the Manga isn't ending in 5 (now 4) years. By Wano's end, over 450 chapters will have been dedicated to 2 Yonko, their crews and a vast mixture of culture and island inhabitants. Now compare that to the ridiculously huge amount of actually important characters, plotlines and endings that need to be achieved for the series to end.
Edit: Funny enough, @Captain_M, I did see your post on reddit lol, and I agree wholeheartedly. Its very likely the final "Saga" or "plotline", rather than a singular "arc". Its where everything will likely collide and come together.
Does anyone really think Blackbeard or Akainu will receive less focus than Doflamingo or Kaido? I'm? Vegapunk? Kuma? Dragon? Garp? Sengoku? Gorosei? Etc etc etc.
There is a scary amount left, to which I wouldnt put it beneath shuesha for advertising the final "arc" or "saga" when in reality there is still hundreds of focused chapters left.