Kishi's not even writing the whole movie, he's just supervising it. He created the character design and plot, but he did the same for RTN, which wasn't canon.
Where does it even say that this movie "connects to the original work"? That actually doesn't even prove it's canon to the manga because RTN was connected to the story as well, and that's not canon either. I want hard evidence not just words from a member.
Also, why would they make a movie completely spoiling the anime only viewers before the anime is over? That's not a good move. I think this movie is just a "what-if" placed in the future, but not actually a continuation and conclusion to the manga. The manga will have its own ending.