This story strangely both sticks to the rule of only leveling up twice, and that's the max, and then mentioning exceptions one can use to keep going. Yang Kai naturally already used one of the exceptions, the world fruit, but there have been others mentioned. So, that only twice is only true for average sect members eating soft rice all of their lives, not for someone seeking their fortune out there, risking life and limb. Nobody's risking more than Yang Kai, so it's obvious he's not going to be stopped by anything, unless the author is a hopeless liar.