From the article: "[The study] demonstrated that rats with access to high-fructose corn syrup ... gain significantly more weight than those with access to water sweetened with table sugar, even when they consume the same number of calories."
So, they did indeed count the calories and the result seems to indicate something about HFCS encourages rats to gain weight in a way that standard table sugar does not.
Your quote is from the caption, the actual article gives no indication of caloric intake.
More importantly, you are just repeating the line that high fructose corn syrup causes more weight gain. So what? There is no indication that this is a bad thing.
So, they did indeed count the calories and the result seems to indicate something about HFCS encourages rats to gain weight in a way that standard table sugar does not.