How can you say he doesn't need a teacher when he was taught red like style and every time he defeated one of those fighters he was taught their recipe.
We know at one point he was caught because he was showed wearing a dress and acting like them. Check out the cover series story at the beginning of chapter 544.
That means there is a strong possibility during that time they taught Sanji their fighting style and haki.
I I have been thinking about it and I will say that maybe Sanji allowed himself to be "caught" so he could benefit from the training. maybe he beat all of them and they were still more of him alone so he had to go through some type of initiation before they would teach him. Like Zoro, there's still a lot We don't know about his training.