They were looking to see if there was some adaptation to losing weight whereby more calories from digestion were being absorbed.
That is, does the post-obese individual's body absorb more of the calories that they are eating. The answer is that they do not.
And even if they did, the difference wouldn't explain the results. Because human food absorption is already exceedingly efficient (e.g. 90-95% for protein, 97% for fat, variable for carbs). Even if the obese did absorb more calories from food, it wouldn't amount to that significant of a number, especailly while dieting.