The entire point of the attack is that it isn't focused so Garp gotta connect the punches directly which isn't easy.
Worst base scenario, even if the attack doesn't KO Kuri, it gives Garp an opening to land a follow-up attack, which should end the fight.
I get why you think Garp outspeeds Kat but like I have a hard time backing that up with feats.
It's not that. It's that we don't have a common ground to make a comparison. We know Kuzan himself is a speedster in his own right. Garp was fighting the strongest people in the verse with the best skills, so he should be able to handle almost any kind of opponent. Even Rayleigh has experienced against FS users.
The only reason why we think Katakuri's impossibly fast is because he outclassed Luffy. Funny thing is, people also call Luffy fast coz he could keep up with Kuri. We simply don't have a metric to make a comparison b/w Luffy and an admiral. Forget admirals, do we even know Luffy is faster than Speed mode King? So I don't see why reason to think Kuri and Luffy represent the pinnacle of speed in the OP verse. If anyone, it should be Kizaru. And I am sure all admirals and Garp can keep up with his speed, which means they should be able to operate at Luffy/Kuri level speeds with no issues—especially if they have advanced CoO.