If a half dead Old Garp can casually speed blitz Kuzan, then it should be pretty obvious that Kuzan would need to give it his all to beat him
And Kuzan was literally showing concern FOR GARP when Garp blitzed him. You know? Being completely distracted and not trying to fight? If an enemy shows concern for you in the middle of the fucking battle, it means they are not trying to kill/beat you. At least not as best they could.
Garp saying Kuzan was soft and wavering > Kuzan lying to BB to earn BBs trust