Squard's stab had nothing to do with anything.
Prior to the illness, they were on even ground, deflecting each other's attacks. Had the fight continued, no illness, it'd have been a slug fest. The illness dropped Whitebeard down enough that they were no longer on even ground and let Akainu hit a clean, direct hit with no defense or attempt at avoiding by Whitebeard.
Whitebeard didn't fight Blackbeard's crew, Whitebeard got killed by Whitebeard's crew. They killed him by just shooting at him and stabbing him, basic tactics. None of them were using powers aside from Teach in s brief skirmish.
Akainu on the other hand was able to fight all the WB commanders minus two, Crocodile and dozens of his pirates and came out without a scratch. WB got killed by some up and coming pirates and some escaped convicts, while Akainu left a clash with dozens of people including WB commanders with no further injuries but WB left their clash as better off? Please.
Kizaru stopped WB's bisento with one leg and injured him twice. Kizaru and Akainu did the same thing when they fought WB, but Akainu is the only one who can fair against Yonko? I don't think so.
Whitebeard with his entire crew didn't do any significant damage to the Admirals. Kizaru remained unharmed, Kuzan suffered one blow that only made his lip bleed, and Akainu was blindsided and dropped in a hole, but he reemerged with no sustained injuries and fought just as well as he did before. But yeah, WB just babyshakes all three by himself