I personally know several people who were not in that 1-5% who did go to MIT and most definitely have massive student loans. Maybe they do that now, but they definitely didn't in the past.
Agreed, there could be people outside of the top 5% who have loan payments, and those from families earning more than $200,000 who decided not to pay the full amount.
With this pledge, maybe MIT can bump up the free tuition level for those from families earning under $80,000 to $125,000 like at Stanford. The number is somewhat arbitrary and depends on the sliding window of tuition rate. After all for well-endowed, prestigious schools, tuition is unhinged from services received / amount of spending per student.