The problem with this not being based on what you know of the eight tails, is this: take away the last few panels, and you'd have the same conclusion. Knowing nothing of the demon other than it's tail number, nothing of the Jinchuruki who controls it, nothing of where he's from, nothing of what he looks like, and certainly nothing about his techniques or fighting ability, you reach same conclusion. He is the strongest. This amounts to you reading the writer's mind. He's strongest because he's last. He's strongest because it builds suspense. Not very compelling arguments I must say.Not by themselves anyway.