Eh the scene in Marineford is suspect but Sakazuki and Kuzan likely just got stronger during their fight by awakening their fruits.
The only thing Shanks did is block a basic punch after which Akainu couldn't act because Sengoku is calling the shots.
I'm not sold on them having gotten stronger. Maybe Akainu by a small margin but probably not Kuzan. He had his will and body crushed.