The letter explicitly calls out that undergrads may request an exemption so that they can stay on campus.

  - International students who have concerns that they would not be allowed to return to MIT due to visa issues.
  - International students who will have difficulty returning to their home country if it has been hard-hit by COVID-19.
  - Students who do not have a home to go to, or for whom going home would be unsafe given the circumstances of their home country or homelife.

(just cutting down the dorm population by 90% will have a huge effect at slowing the disease, so I imagine there'll be plenty of room to give exemptions for students without safe/stable homes to return to)