I suspect that map is full of lies.
Look right over Jacksonville FL, that patchy area. Thats I-95. If they can't have full coverage over the most traveled interstate in the nation, they can't possibly be covering rural Texas, the middle of nowhere in New Mexico, half of Nebraska, the Louisiana Bayou, the woods of Northern Michigan... etc. And thats just a few mentioned off the top of my head.
I've traveled each of those places (except the Bayou), and spent hours zoomed in on Goog Maps, looking at little towns and rural villages, checking the populations in Wiki. In those places, there is literally nothing. It would take an hour by car to get even
close to a place where it would make sense to provide coverage - yet they color it orange.
I call bullshit on that whole map.