Differential GPS is needed to achieve this accuracy. At some point in the future, vision systems + coded lighting may suffice, but for now, more elaborate tech is needed to keep these machines in tight formation.
I don't think differential GPS does anything here. It corrects for large-scale ionospheric delay error, which is going to be exactly the same for every drone at this scale. And they have WAAS receivers anyway, which provides better accuracy than differential GPS. (Maybe you mean RTK, which could help achieve centimeter accuracy.)