How do packet cake mixes get their lightness without creaming butter and sugar?
Cakes pretty much always have chemical leaveners in them, whether you're making them from a mix or from scratch. So yes, there are chemical leaveners, and that's plenty to get it that light. I'm not even sure if it takes significantly more leavening than you'd need if you'd creamed butter and sugar, since cake batters are wet, so sugar will generally dissolve more and not provide a structural component. Some cake recipes don't even bother with a creaming step. In any case, it obviously works. Creaming butter and sugar is perhaps more of a factor in cookies, as mentioned in this question, for example.