Student or Learner
This "they" is translated as "people", but I think it refers to "settlements" which means a place for settling. Also, considering the passive voice of "be moved", it must be a place not people. What do you think?
ex) High up in the Andes is grand Lake Titicaca. Located at 3,825m above sea-level, it is one of the highest lakes in the world. Over 190km long and 80km wide, the lake is home to hundreds of descendants of the Uro tribe. The Uros settled at the lake during the 16th century to hide from their enemies, the Incas. For centuries, the Uros have lived in reed huts built on artificial islands also made from reeds.Each year, more reeds are added to replace the rotting layers below. To prevent the islands from floating away, the Uros use ropes attached to wooden poles which are driven into the bottom of the lake. The purpose of the island settlements was originally defensive; if an enemy came, they could be quickly moved.
Yes, the settlements could be moved quickly.