Dude Kabuto never said that Hiruzen was the strongest Kage at the time of his death. Because that is as untrue as untrue can get.
If need be I can list the kages who were alive when Hiruzen died.
Hiruzen loses to literally every single one. At the absolute most he might be able to pull a draw with Mei by death reaper seal, but he loses soundly to Onoki, A, Gaara, and Nagato.
Nagato was a kage by the way, as was Orochimaru. Oro was kage of the Sound, and Nagato was Kage of the Rain.
Kabuto was only talking about Hiruzen in his prime. In fact he said "in his era".
And even that is debatable, highly debatable as I don't see how Hiruzen would hurt the third raikage at all, or a younger Onoki who would have more chakra and less back problems than he does now.[/QUOTE
This is a argument between you and the writer, if he states that his character is the strongest kage in his prime, then that's the way it is. The reason why you can say all these about the other kage is simple, they gotten stronger since the 3th died and kishi showed their abilities.
The thing is if you go back those years in Sarutobi prime, that means the other kage who are stronger now, their abilities would reset to what it was back then, which is why sarutobi would be the strongest. right now every single kage is stronger than him.
Like i said tho a writer would never establish his own character power if he did not want people to believe and understand the character power and role in the story.