They only need to get people along the immediate coast a few miles inland or up to higher ground. No need to actually leave the island. Tsunamis kill when you are not aware they are coming, hence the extensive Pacific tsunami alert network (the Indian Ocean had no such network in 2004, hence the millions that were surprised).
Yeah, no need to leave, but tell that to thousands of panicky tourists who are suddenly very homesick.
Hilo airport just closed....thanks maggie