Luffy had already been fixing the issues of his fighting style and he was much stronger than before. But his fighting style was getting stale. With this it clearly shows you and states to you how Luffy is constantly getting better and better. How he’s always developing and achieving new levels of power. He uses what he sees and learned from his previous fights.
He develops two new original Gomu Gomu techniques. Second and Third Gear. One overaccelerating the speed of his heart for the purspose of achieving constant maximun perfomance. And the other putting air in his bones and using the physics of pressure with liquids to massively increase his punching power (not really how it should look but the design looks cooler that way).
Luffy copies the techniques he sees and is capable of understanding from the guys of the CP9. And simulates the ability of Bellamy by compressing his muscles to multiply his strenght instead of having to make the telegraphed and obvius slow attack he already was doing.
See how this things either fix a weaknesses of the already stablished battle style or potentiate an already existing strenght?
Remember how I said Luffy’s fighting style was slow powerful grounded attacks with long wind ups and uncontrollable high bursts of speed… All with a near infinite amount of stamina?
Luffy quite literally loses his weaknesses and expands on his strenghts by using better the strong suit which he wasn’t really using before. Luffy’s body of rubber could handle insane amounts of work and punishment before tiring. At the cost of his near limitless stamina he could fix his other weaknesses for short periods of time. While still maintaining his skillset. This quite literally builded upon what Luffy already was and what he had learned instead of just making him magically stronger.
Then the time skip did something similar. But this time it literally made Luffy level up.
All of his stats where waaay higher than before after all. But he won them. And this time it wasn’t out of nowhere either. Luffy had lost. He had failed and failed because he already had learned everything about what he already was. He had to build upon new things now. Develop new skills.
The time skip makes it so that it is earned. Luffy did bullshit those skills. They where either taught to him or quite literally he trained until getting strong enought to use them. He learned about the types of Haki which where a tool he was lacking and needed to progress.
He became much stronger physically… Thus almost completely stopped being reliant on amp ups and could use his techniques without repercussions. He developed a new way to force his body to his limit after the other two became a standard… Not out of bullshit but because he already had the knowledge to do it. He just applied the same principle upon a stronger body.
I sincerely love how this series handles getting stronger compared to pretty much all others. It quite literally never feels like bullshit.