Froot Loops have 25.3g of net digestible carbs (total carbs minus fiber) per 30g serving. That's 84%.

http://nutritiondata.self.com/facts/breakfast-cereals/1532/2

I'm fairly certain your link only includes added sugars which is not the total number of digestible sugars (digestible carbs = digestible sugars). Unfortunately, most people have the misconception that carbs from "natural sources" (fruits, starches, grains, etc.) don't effect your blood sugar the same as added sugars, but that is incorrect.

Starches are digested to sugars, and they may indeed have similar effect on blood sugar in some cases, but the article did not say "75 percent carbs", it said "75 percent sugar". "Sugar" usually means HFCS or sucrose, both of which contain a high proportion of fructose, which is metabolised differently to the pure glucose you get from digesting starch. Additionally, there's some evidence that taste influences overeating, and starch does not have the sweet taste of sugar. Japan has the longest life expectancy in the world and their diet includes large quantities of rice, so starch is not obviously harmful.

Does that include sugar substitutes, aka sweeteners?

Sugar substitutes are generally hundreds of times sweeter than the sugar they replace so they wouldn't even be a blip on the composition by mass. This is why cans of diet coke float in water but cans of regular coke sink.

From the documentation:

"Total sugars is the term used for the sum of the individual monosaccharides (galactose, glucose, and fructose) and disaccharides (sucrose, lactose, and maltose)."

http://www.ars.usda.gov/sp2UserFiles/Place/80400525/Data/SR/...

75% sucralose sounds horrid. I can't imagine anyone would want to eat a bowlful of Splenda for breakfast.