I live on that little black speck on the east coast of Florida and I can assure you that their map is wrong. There is no way they comprise that high of a population here.
Looks like Melbourne, Brevard County & Cape area. I agree, I hardly ever see an Islamic-type person there.
I think that's just an coastline artifact; i.e., if you have a lot of zigs and zags in the coastline, as in a fjord, then the whole region is going to look like it's solid black. Southeastern Alaska shows the same effect.