If the drones use e.g. laser rangefinders to measure the distance to 2-3 known reflectors, and maybe to each other, with sub-mm accuracy, that may be enough.

Deploying an antenna that's effectively 50 or 100 m wide by lifting 10-20 drones, after some simple ground preparations, could be invaluable in many scenarios, especially for the military, of course.

The same technique as he uses here for synchronizing the receivers would work. Common reception and triangulation of a strong local beacon would enable positioning at much better than GNSS precision, both because of the SNR advantage itself and because you'd have more options for dealing with carrier phase ambiguity.