"It's the carbs."

As in it's only the carbs. Does a high fat diet make a line of rats fat? Did it have sugar, what what was the carbohydrate content, valid questions. But a combination of high fat and high sugar can be fattening, even when high sugar or high fat in and of themselves, are not. Even if sugar/carbohydrate were necessary conditions for fattening, that wouldn't mean that high fat didn't contribute once those conditions were met.

Of course this is the problem with the gold standard of changing only one variable, it may make it clear that that variable is making the difference, but the result is very specific to the control. There is no "control and intervention" just two interventions with a single variable.