There is no such thing as a "mandatory" evacuation of Ambergris Caye. To require that people leave, you need to be sure there is transportation - and simply put, there is not enough of it (nor money to try to provide it).
Evacuation can be recommended - but that's all.

I live 5 miles north - people are riding their bikes past our place now - they were passing by yeterday also, even though it was pretty difficult slogging along the beaches.
The pull-ferry is in operation.
I saw only one boat go by today - being off the water is still a really good idea.
VERY brisk winds now, not much wave action at all. Yesterday was the opposite - high water, mild wind.