But there doesn't seem to be carbonated water?

I think the assumption is that CB and Diet CB, would be something like coke or orange soda or whatever, and that degassed CB would be just coke that was left out so the bubbles dispersed. (Rather than 'soda water' as carbonated water might be called in some places).

The obvious experiment would have been to test tap water vs. carbonated water, which makes one wonder if this study is yet another example of p-hacking. (We were testing this other theory and look at what happened in the dataset, let's make a paper on that!)