Those just seem like bolides to me. If a craft were able to accelerate through a fluid as quickly as described by Favor, the resulting boom would be like that of a lightning bolt thirty feet in diameter dozens of miles long, and the burned air in its wake would look about the same. So if we pretend that his account is accurate, the craft must have some way to deal with fluid viscosity that is not understood, just like its propulsion is not understood.
This is something I don't think has really been addressed, or even speculated upon, and is more scientifically difficult to believe even than the magic Element 115 levitation stone. But if you're believing one thing, might as well believe another.