He ran CPS for six years.
Mr. Duncan did far more than create charter schools.
According to Mr. Duncan's archived DoE biography, Mr. Duncan had an "aggressive education reform agenda that included opening more than 100 new schools, expanding after-school and summer learning programs, closing down underperforming schools, increasing early childhood and college access, dramatically boosting the caliber of teachers, and building public-private partnerships around a VARIETY of education initiatives. Duncan is credited with significantly raising student performance on national and state tests, increasing graduation rates and the numbers of students taking Advanced Placement courses, and boosting the total number of scholarships secured by CPS students to more than $150 million." (emphasis added)
The DoE staff undoubtedly dressed up Mr. Duncan's CV (just like they will for Ms. DeVos), but I doubt he did that solely through Ms. DeVos' charter schools.