Luffy had a significant advantage over katakuri as having fs nuffies kats main abbility/strategy.* Addtionaly snakeman is designed by oda to counter katakuri. It counter his main strategy of doging everythingby it attacking in weird ways and everytime an attack is doged it just it increases in speed. Despite this katakuri manges to blitz it twice and dodge most of its attacks (slide 5&6). Addtionaly katakuris haki was pikely significantly weaked by the time he fought snakeman as that eas luffys enitre strategy (slide 1). Luffy was also saved by plot armor multiple times in this fight as katakuris weapon which can inflict a fatal wound every time it hits (slide 3) just randomly disapaers as well as the fact that in this fight he is able to survive lethal hit after lethal hit and contiune to fight(slide 2&4) which is an endurance feat he never replicates again. 

*I know somone is gping to rely saying that luffys fs was counter to so its an equal nerf to both side when that obviously isnt true as 1 katakuri is significantly better at using fs than luffy (who sucks at using it) 2 even if luffy was perfect at using fs he still wouldnt be able to use it anywere close to as well as katakuri as after the first dodge his opponent can react in a way he didnt forsee(as fs doesnt show the alter future) while katakuri's effective dodging leads his opponents to doing the same thing as he forsaw meaning he can dodge every attack with fs while luffy can only dodge 1

Anyone who this katakuri~end of wci luffy never read there fight