My favorite thought experiment is to take someone at 2000 calories a day of a standard balanced diet and reduce them to 1500, but change the diet to pure sugar (with some vitamin supplement so they won't die of malnutrition). Will they lose weight? My guess is they gain weight, because the types of calories matter. Mere restriction of calories isn't good enough.

This is simply factually incorrect. Didn't you hear about the guy who lost 60 pounds by eating nothing but McDonald's? CICO, if properly followed, works.

If someone really, truly eats 1500 calories of sugar per day--assuming their maintenance calories are somewhere north of 1500--then they WILL lose weight.

But what will likely happen is that this person will find it next to impossible to stick to this diet, because they will constantly feel hungry. They are very likely to cheat and eat other foods, or binge on more calories than they will report having eaten. The vast majority of the time when CICO seemingly doesn't work, it's due to human error--whether intentional or not.

If you could ensure complete accuracy of someone's diet, a calorie deficit would indeed lead to weight loss. (Note that they will lose weight, not necessarily fat.) Taubes doesn't dispute this, but he does like to downplay it to an excessive degree.