They've addressed this many times. There are different "grades" of maltodextrin with different glycemic indexes. Soylent uses a higher grade with a low glycemic index. Google "low DE maltodextrin." Soylent also uses other low-glycemic carbs such as isomaltulose.
The glycemic index of a carbohydrate is a bad measure of its quality. A better measure is the degree to which it is fermented by bacteria in the intestines. Furthermore, there is research demonstrating that carbohydrates consumed without the phytonutrients that normally accompany them in whole food increase markers of oxidative stress and inflamation.
Soylent might be better than a frozen pizza or fast food, but in the big picture it's still shit compared to real food.
Hm, so for someone who eats frozen pizza and fast food on a daily basis (because of the convenience), would Soylent be a step up from that based on your comment?
Absolutely, but for just a tiny amount more work you can throw a bunch of lentils, frozen vegetables and meat in a slow cooker and get very close to an optimal diet.
I find I have to combine it with a fat/protein like a serving of peanuts to make it not spike me hard. It may not have a high glycemic index, but it is already in liquid form and is thus available for digestion.