I agree with your doubts - I don't think we do really know. IIRC this subject all started off following a post in wnxx by Tony Miles, following his comment on Radio 4, which referred to 15 units. That eventually was reduced to 9 units.
I think all we do know for certain is that the 168 extra vehicles for NXEA (30 x 379 and 17 x 321) were organised and agreed with DfT back when it was intended that 12 car trains would be needed on Stansted Airport runs.
Clearly this isn't happening now, and a number of posts have suggested (across a range of forums), and going back a year or more, that this has resulted in NXEA having a certain amount of spare capacity especially if all the various out of use units were reinstated.
So all in all I think you can only say that if the excess capacity is on the WA and Stansted routes it is probably 317s that become available.
However the December timetable was written after it was decided to run the Stansted Express as 8-car, and it has additional services, services using 317s instead of 315s and services that are 8x317 up from 4x317- so quite where that leaves fleet availability I'm not sure- with the intention of using the 317/7 for GE services it may have required some of the "resting" units back in service?