Here’s are some pictures that I edited to help visualize what may have happened during the final hits of the fight.
In this first picture, I colored Luffy’s attack in purple and Katakuri’s attack in pink.
I did this to help see the amount of distance that each individual arm covered. King Cobra looked like just an ordinary punch since we really didn’t get a full final panel. But if we look at how much distance their arms covered within the couple of panels, we can visualize what happened.
In this second picture the attacks have already connected, we see the impact that each attack had on the ground, Luffy’s arm and impact damage are purple, Katakuri’s impact damage is pink+green, and the colors mean the same as the first picture.
Notice in picture #1, Luffy and Katakuri are fairly close before unleashing their attacks. Katakuri’s arm is short and then gets very big quickly in this panel. In picture #2, we see Katakuri’s arm stretched out a lot farther than what it was when they were closer together in #1 unleashing the attacks. This means that Katakuri was pushed back the quite some distance. He needed to hit Luffy after being pushed back so he stretches his arm in able to slam Luffy into the ground.
Also, notice the impact damage made from Katakuri’s arm. This is important because impact damage isn’t only shown from Katakuri’s attack it’s made from Luffy’s as well (King Cobra’s impact damage/purple). Whether it was due to Katakuri sliding across the ground and his feet caused it to happen, or King cobra ripped up the ground as it passed, it means King Cobra hit Katakuri very hard either way.
Hopefully this helps some who had trouble realizing what had happen. It’s weird that we didn’t get to see the final blow but we gotta work with what we got.
*Theory by dontblazemebro