Like you did? Where does Kizaru say in that panel that he'll defeat Kaido and Big Mom? Nowhere.
These are again false equivalence, these statements are made by uninformed characters before they fought those individuals, Kaido already fought SaturnMu before making that statement, what aren't you understanding about this?
That statement by Akainu is nowhere near as direct as "nobody in this world can defeat me", again, just yet another false equivalence.
If Akainu had fought SaturnMu and then said "nobody in this world can defeat me", yes, I would absolutely put Akainu > SaturnMu, that goes for all characters.
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Good thing we aren't talking about real Imu, just SaturnMu, so Saturn as he was while possessed by Imu.