I think it is more likely that it was assumed that DJI (among others) would be able to make firmware updates that would enable RID on the drones...I know I asked about RID here on the forum last March and all the responses I get were to wait for DJI to make the announcements and update my firmware......it never happened...so the FAA was not the only one caught off guard.
Overall, if you consider the range and scope of the responsibility of the FAA, they are actually quite well run and organized.....I wish they had the authority to make airports have a clean area to sit down and put your shoes back on after going through that preflight check on commercial flights