Actually, fighting Aokji is the perfect reason for Akainu to end up a tier above the other admirals.
We know that Haki can bloom in extreme diff battles so Oda has a good excuse to give a new PU for Sakazuki, like adv.CoC. I mean, they both were fighting for the leading position of Navy. Also the former FA had that power as well so the portrayal fits well.
I personally don't care much about powerscaling and I like to look at things objectively, considering all of: hype, feats, dialogue, worldbuilding and even possibilities - but defeating Kuzan doesn't have any implications at all and here is why:
The problem with your argument is as follows:
-There is an implied upper limit to the strength of Haki for every single user
-The Admirals are veterans in combat and they are all in their 40s or older unless Aramaki breaks the mold but it seems unlikely
-It is therefore highly likely the Admirals are maxing out, or at least approaching the maximum potential of their own Haki
-Furthermore, who's to say Borsalino, Issho and Aramaki aren't actually stronger than Kuzan for example? There is nothing canon that opposes this suggestion or proves Kuzan is stronger than any Admiral at all
-You are interpreting the development of Haki in combat on the basis of an upstart crew who are all young, new to Haki and developing at an insanely fast rate, not on the perspective of normal characters and veteran Marines
As I said, in all likelihood yes Sakazuki is the strongest. However if we were to attach numerical value to his power (say idk, 100, just for arguments sake) then it is also implied that none of the Admirals would be below a 98 and this is just the pre TS admirals. As Sakazuki's promotion predates the draft there is every chance Fujitora and Ryokugyu could be as powerful or above Sakazuki in individual power. As i keep repeating, there's no way of knowing at all. All we know is that it is more probable for Sakazuki to be more powerful than Borsalino than the other way round. That is the absolute limit on what we know right now.