Since WB failed hard to pwn or even harm Admirals when he tried attacking them in a fair fight.
The moment WB stepped down from his ship, nothing was fair anymore. One of his alliance-mate was played and WB ended up with a hole in his chest. I don't need to mention that before the vs Akainu fight, WB was multiple wounded by the marines. While Akainu was fresh, without any signs of damage.
That battle was no way in hell fair. To me a fair fight means if both fighter fights on neutral battlefield, without any disadvantage for each and naturally both of them have to be fresh, without damages, exhaustion etc.
Yes Akainu was able to take half of his face, but WB was in a hurry. While for Akainu it was just
vs WB, for WB it was more like
vs MHQ. He needed to buy time for his crew's escape, while he was holding the entire MHQ. So he couldn't focus only on Akainu.
Now imagine, if it wasn't MHQ but just Akainu, don't you think WB would have jumped on and focused more attacks on him, even after he felled down? So not just 2 hits, but more, way more. I think it is pretty clear that Akainu couldn't fight anymore after the 2nd hit. That's why I can't imagine him surviving further hits, while WB who was shot and stabbed several times, who also lost his face and had a hole in his chest, was still not down.
The way I see it, WB wanted to take down Akainu as fast as possible (reasons: Ace and Crews safty), so he let him take his face, but landed two fast hits. Under normal condition the WB vs Akainu fight would have lasted way longer, WB wouldn't have lost his face in a few seconds and Akainu wouldn't have been defeated so early.
WB vs aynone displayed so far = WB -> the strongest man
WB vs 2 Admirals = Admirals
Anyway, it's not a secret that Im a WB-fanboy, but Im not pulling my arguments out of my ass. He portrayed his superiority, not to mention Oda did not make him lose in a fair 1vs1 fight. So I believe with that infobox labelling him as the strongest, was not a title, but rather a fact.