Its long been understood that soda w/o carbonation isn't palatable because its too sweet and animals stop consuming it. However, when carbonation is present it masks the sweetness which result in increased consumption.
So I think they're probably measuring a secondary effect here.
This controls for the concern you raise above regarding palatability of the beverage, so I don't think that can be a factor there. Critically, there's a statistically significant difference between water and RCB, but not between water and DCB (though the ghrelin level was higher in DCB than water).
From my reading of the paper your concern could certainly be relevant in the rat experiment; I couldn't see any attempt to control for or measure the amount of liquid that the rats consumed, so they could have been drinking less decarbonated beverage.
Soda without soda? :)
As a non-native speaker, this continually fascinates me...