So according to the article:
100% of college students who’s politics changed in college became more leftist.
Out of every 10 college students, 4 become more leftist, 6 don’t change, none became more conservative.
How does this lead to the title “Professors don’t rub off politics on their students”?
But you missed the phrase ‘mirrors 17 to 24 year olds generally’. The non-college population also shifts left during those years, and serves as a control group.
If the professoriate was successfully indoctrinating their views, one would expect more of a shift left among those in college than among those not in college.
Remember Churchill’s dictum “If you’re not a liberal at 20 you have no heart, if you are not a conservative at 30 you have no brain.”
Early 21st century America’s extended adolescense probably shifts the relevant years about 5 years later in life.