They say it works "by directly converting electrical energy into thrust through controlled electromagnetic impulses", so I assume it's reacting against the Earth's magnetic field using the Lorentz force?
Yes but they also don't seem to claim it does. The use cases they talk about are orbital station-keeping and de-orbiting of satellites. So that implies near-Earth use.
Their patents don't say anything about an external magnetic field, so at best they've accidentally rediscovered it through experimental error and don't know it yet.
I suspect that you’re right, but I believe that I heard something about a similar ‘drive’ being used on some sort of telescope satellite in the past (though I can’t remember the specifics).