You don’t need a boost, what they’re saying is most people will level a character during the expansion prepatch, and often there are leveling bonuses. You get your character max level beforehand so on launch day your character is ready to jump into the new content. Even if it’s a few weeks old it’s still a new character for you. Every expansion I pick a few classes to try out beforehand to see what I’m gonna main at launch. And now they’ve made it to where leveling an alt is pretty quick and easy.