It has no useful relationship to reality. It relies on a self-selecting group of people to download and run their app to collect data, and then presents that data in what I'm sure is a precise but utterly inaccurate form.

If you believe it, there is no signal from any carrier at either my current home in Washington, nor my previous home in Santa Clara. And there is certainly no T-Mobile, AT&T, or other GSM carrier signal to be found anywhere near my current home (my AT&T iPhone stubbornly insists on working regardless).