I've not used a calorimeter, but my understanding is that is just measures heat energy from burning things. Things like sucralose and aspertame likely WOULD read as caloric in that kind of measurement, because they contain chemical energy. The reason those sweeteners read as 0 in nutrition labels is because of how we metabolize food. Or in this case, how we don't: we can't digest sucralose and similar sweeteners. It goes in, your tongue says "yay!" and you poop it out.

Also a schmuck on interweb; but healthcare is my area of expertise, so I've got a handful of college level human anatomy & physiology, nutrition, and microbiology courses to draw from here. I was also a fatass who wanted to join the military back in the day, which required losing a lot of weight - decided to approach it as scientifically as possible (there's a LOT of fad misinformation surrounding weightloss), and I can't complain about the results.