Did you book on southwest.com or directly with Avis? That cached snapshot of the offer page has both the AWD and the coupon page pre-filled. If an older version of the page sent you over to Avis to book there may have been a problem with the URL on southwest.com. I know I ran into an incorrect rental car URL on some partner site, one that contained one of the codes and not both; it may have been the RR 5x Avis offer. Here's an example of a correct URL from costcotravel.com:
https://www.avis.com/car-rental/profile/bridge.ac?ACTION=1&ACCT_TYPE=2&SEG_ID=20046&AWD_NU MBER=A108364&COUPON=MUGA049&CID=A108364_35offweekly
If Costco left out the bolded portion of the URL, the coupon code field would not filled for you. SWA isn't known for attention to detail, so it is certainly possible they did not include a correct URL. I don't think the 5x RR offer had a page on avis.com, so if you went there via a link on a southwest page, the failure to include the coupon code was SWA's fault, not the fault of Avis.
Do you recall roughly when you made the reservation?