This is an abstract of an experiment in progress, not a peer-reviewed paper. A search of Pubmed reveals no further paper on the subject, even though the abstract is over two and a half years old...
...which should tell you something about whether that particular hypothesis turned out to be correct.
Yes, I'm pretty sure Coca-Cola made with HFCS doesn't magically have 5x the amount of calories it says on the label due to "undigested cornstarch". There's 39 grams of either cane sugar or HFCS in a 12 ounce can of Coke. If there were really 5x the amount of carbohydrate, that would be 195 grams of carbohydrate in a single can.
Note that 12 ounces = 1 1/2 cups, and cornstarch weighs 125 grams per cup. I'm pretty sure there isn't OVER A CUP AND A HALF OF CORNSTARCH IN A CUP AND A HALF OF COKE.