it's called Sulong, you did read the chapter right?
Because he is OP he is on a level Luffy is not supposed to be anywhere near yet, for him to just out of nowhere with no training or real ace up his sleeve beat Katakuri would be bull
Luffy already received 1.5 years training by one of the best, if not the best, Haki users in the world. That person left Luffy alone for the next half year because he believed he couldn't teach him anything anymore and that everything else is up to Luffy himself. That same person also taught him that the stronger the opponents are Luffy fights, the more his Haki will increase.
I've said this before, but this entire conception of people trying to create a natural pecking order among characters and using common sense to formulate some kind of universal training regime that everyone could follow to become the strongest, is misplaced. Then you are reading the wrong manga.
This is just my personal opinion, but Dogtooth has been fighting for two-three decades by now. Luffy a fraction of that, so rationally speaking Luffy should spend at least a decade or so training in the New World to have a chance against him (which is not going to happen). So I find it a fallacy to say that Luffy isn't anywhere near Dogtooth, but if he takes a time-out of a few weeks or months, somehow that would change everything? That's a farce. Then you are trying to provide a rational explanation as to how Luffy possibly could beat Dogtooth, however it's with a shallow facade. I rather prefer then that Luffy beats him here and now because at least it's genuine.
Besides this is the New World. Common sense doesn't really apply here as if there was some common training way to be able to stand up to Yonkou, everyone would be doing it. The New World is a place of monsters and irrationality and where your spirit will make the difference. The stronger your opponents are, the more you will have to push your Haki to its limits and thus the more it will be able to grow. That's what Rayleigh explained to Luffy. It's highly dangerous and the majority of the characters would never succeed, but the few ones who do will make giant leaps and that is something Luffy understands. He considers it imperative to beat Dogtooth right now as there is no alternative. No amount of training would be an equivalent.
Imagine Luffy is able to flee from Dogtooth and this arc comes to an end. They are then going after Kaidou, so somehow he has to succeed at that when he even failed at moving passed Dogtooth's threshold? No, Luffy kinda has to beat Dogtooth and he knows it.
This is just an extension of the premise of the New World that was set early on: either you join a Yonkou or you fight them. What Luffy and the others are doing right now is their training.
