I don’t understand the fuss. First off, there have been dessert recipes that call for soda for decades. Cracker Barrel serves a coca-cola cake, just as one example. The soda contains sweetener and the carbonation helps the cake rise. Second – and since it may have escaped her notice, I’ll be blunt – IT’S CAKE. Even without the Dr Pepper, it’s not meant to be health food! It’s not like if you leave out the soda this would suddenly become a slab of quinoa, kale, and acai berries.
It seems to me that the use of Coca Cola or Dr Pepper is just a really-overpriced sugar-water ingredient. I don’t buy the idea that it helps it to rise because the carbonation is probably lost as soon as the temperature rises high enough.