No one attends Harvard 100% for free. Even with literally zero income and a bunch of kids in college - the most pessimistic outlook their NPC allows, the student contribution is still $4600 per year - $1600 summer earnings and $3000 work study.
However, if a student earns outside scholarships, Harvard will allow them to use those to reduce work study and summer employment expectations - so if a kid can get $4600 in outside scholarships, then they can pay nothing.