I would guess that their overall simple carb consumption is way lower than other cultures who have less atherosclerosis (Sweeds). It could be the saturated fat afterall, the lipid hypothesis is false but there is a correlation between ldl's and saturated fats (I think according to Berardi) and the Masai probably consume more saturated fat than any other culture.
I've heard that the Swedes have high rates of atheroschlerosis close to the United States because of their sweets consumption. IMO, for the Masai, it's the high sugar along with the high fat together. I believe Berardi mentions that carbs and fats are not good to have together. That could possibly be an argument for skim milk instead of whole milk. And high LDL cholesterol has never been confirmed to cause atheroschlerosis. That is also a theory/hypothesis.