It’s just basic logic.

Sakazuki and Kuzan fought for 10 days.

So we know Sakazuki >= Kuzan. It’s also pretty obvious that Sakazuki is the strongest admiral, which clearly gives:

Sakazuki >= Kuzan >= Borsalino.

Since Kuzan being so close to Sakazuki would bump him above Borsalino.

And logically we know that the Navy is the strongest it’s ever been. So the two new admirals have to be at least equal to Kizaru. Which gives:

Sakazuki >= Kuzan >= Borsalino = Issho = Aramaki

Almost a proof if you ask me.
 
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