They talking about having the title of strongest, not being strongest? Or maybe fastest in speed. Nagato wasn't acknowledged as the strongest, no one knew anything about him since he in Akatsuki. The Kages are the strongest because they are known by everyone for their feats and strength even outside their villages, that's what Naruto is aiming for and he needs to surpass his fathers fame that he had at that time.
Put this together really crap since its so late but that's how I see it.