Here's some evidence to throw on top of the fire:

I signed up for the promo the day it came out. Earlier on the same day I had booked a roundtrip within CA via my corporate booking site, Axiom. Axiom can not automatically add RR numbers onto the PNR. After I had signed up for the promo, I attached my RR# to my PNR.

Since I no longer seem to be automatically receiving my flight credit anymore (separate thread), this morning I applied for past flight credit and received 2 credits for each leg!

So what does this show:
1) It is possible to receive the promo for flights booked prior to signing up for the promo. I will temper this by saying that the flights were booked on the same day as I signed up and adding the RR# after signing up for the promo may have had something to do with it
2) Flights do NOT need to be booked directly with WN to be eligible. The T&C's seem to imply this because they state that RR# must be provided at time of booking which isn't possible with Axiom.