Those posphonate groups will hydrolyze pretty easily with stomach acid. All of the hydrolysis reactions are highly exergonic. In the cell, the pH and environment is not very extreme so ATP can last, but I find it very unlikely that ATP can survive stomach acid. The only way ATP could survive is if there were some unforseen kinetic barrier like sterics.
Aren’t calories measured by burning the item and recording the heat produced?
In that case something like C4 would have a very high caloric density.