He's speaking in reference to the "obesity" epidemic. Lustig is postulating that obesity rates would not be as high as they are if it were not for the massive sugar consumption in the western diet. Taubes agrees based on the science that is out there.
This is not to say that people who have damaged metabolisms or who are naturally insulin resistant would not still be obese if they cut out sugar from their diet. Carbohydrate consumption in the form of glucose can still lead to the insulin rollercoaster that drives obesity. But would fewer people be fat if sugar was not so prevalent in diet? Most likely.