I feel like the ending was definitely ambiguous, but it's kind of like... The atmosphere of the manga has this sense of fatalistic sincerity about it, and I feel like it would be out of character to start playing tricks with the audience's perception in the last few pages. It's not that coy, so to speak. If it was going to give Kaguya an eternity of suffering comforted only by her delusions, I feel like it would be happy enough to just do that. I think we can trust the ending to be honestly presented, if only because it would have no interest in lying.
Also, framing convention. The last shot being focused on one character's expression as the final events of the story happen offscreen is recognisably classic Japanese cinematography, and it seems to be played pretty straight here. We don't actually see her ourselves, but that's more for ~subtlety~ and a sense of ~This moment belongs to them alone~ than anything to do with us.