Gardner, your presumption is correct. It's not in the T&Cs of the offer because it's in the general T&Cs which require us to declare the FF# of one airline/ hotel co. of our choice for each rental.
Because the free weekend does not require a coupon # , only registration, the DL coupon # may be entered so we can earn 5K DL + a free weekend rental ! Too bad the bonuses for other airlines are much smaller.