First, BC is not light. Most applicants at most schools, including MIT and Caltech will have had BC or less. The vast majority of students simply don’t have the opportunity. The CS curricula at both MIT and Caltech presume Calculus during 1st year.
Certainly there will be students at both who have had more math. I don’t know that it is an advantage in and of itself. What seems to be is having a background in math competition and doing well.