Not putting this lightly, but only two of Grand Bahama's islands were seemingly destroyed.
And they sure as hell had plenty of warning to get to higher ground somewhere.
I think the impact is difficult for many of us to imagine. Dorian was huge. It basically affected every island in the Bahamas. We hear about damage in Nassau and Freeport because these are the cities most of us know. But the other islands -- some of which perhaps are not as populated or maybe even unpopulated -- were also hit hard.
As for evacuating to higher ground, others here have pointed out there really isn't much in the Bahamas. Cat Island has the highest point -- 207 feet -- but the more heavily populated islands are much lower. I'm getting conflicting reports of how high the floods were, but one said as much as 23 feet! For islands which average 30 or so feet at their highest point, there really isn't much in the way of "higher" ground.