I haven’t been able to find documentation on how it works but as they claim you can do 4G in the middle of the ocean, it can’t be ground stations.
The issue is potential interference with terrestrial mobile networks, particularly those in non-approving countries and of non-partner operators. A test over the middle of the ocean may not violate ITU Radio Regulations, but a satellite transmitting over land in a way that interferes with a mobile operator’s network surely does. I looked at some of their recent FCC filings and they seem to say they will be able to manage this using very tight spot beam control. (They will have to work around arbitrary national and operator license borders.) Other than that, they decline to say how this will work, drawing on the theory that they are just a contractor for a terrestrial service who remains liable if they mess up. I’ll be interested to see if that argument flies.