Except that the Vasudan scientists on Altair didn't seem to know who the Ancients were when they described their find. "These ruins are neither Terran, nor Vasudan, nor Shivan."
So most likely, the Zods developed subspace drives indigenously. Possibly even before the GTA did (they had the motivation of needing resources from offworld).
Either that, or they had to bribe/steal from the Terrans (which is unlikely IMO, since if that were the case, the Terrans might have overrun Vasuda very quickly right after the war began, simply due to the massive tactical advantage granted by subspace technology).