Interesting, but the experiment there seems to be flawed. Cookies have a lot more sugar/glucose than radishes and it changes your blood sugar levels a lot more, so you'd expect right after eating the people that ate cookies will have more energy to persist on the task.
It doesn't quite prove what they are trying to prove (that self control and change are exhausting). They should have tried it with something else that was tempting and that was not food.
There's another study (that I can't find right now, maybe someone will help me) in which self-control was measured by how long someone could keep their arm submerged in icy water.
Ones who had their discipline taxed before the exercise performed dramatically worse than those who hadn't.
So that one had nothing to do with food and showed similar results.
Good point. A simple fix would be to figure out what a normal score on the exercise would be without cookies. If it's the same as with cookies (or close), it's safe to conclude that it's the temptation of the cookies moreso than the sugar that makes the difference.
Your second sentence sounds like you actually agree with the study. The people with heightened levels of blood sugar pursued the impossible task a lot longer than the people with lower levels of blood sugar. Determination in the face of frustration is an important part of self-control. It sounds like they've showed such determination is tied to blood glucose levels, which certainly do get depleted after difficult tasks.
It doesn't quite prove what they are trying to prove (that self control and change are exhausting). They should have tried it with something else that was tempting and that was not food.