Haiti is the poorest country in the entire western hemisphere. They have virtually nothing to help themselves with. They are totally dependent on outside help except for the muscle and make-do of the first (immediately local) responders.
In a country as severely hit and agonizingly poor as Haiti, the PM is probably right about deaths in the hundreds of thousands. And I'm sure he's not just referring to the ones who die in the first four days of trauma.
Sue