Oneworld ? the proverbial other shoe dropped and explains the haste and timeline of AC's offer for AE. RBC/Westjet/Oneworld explains AE stating that after AC you can redeem on 20 airlines. Oneworld has 13 partners and 30 affiliates.
My guess is that the 20 includes Oneworld, Westjet, Alaska and a few others.
Missing or at least not reported is if Amex and/or CapitalOne are involved somewhere other than RBC. Or maybe in lieu of RBC. Which may explain why Visa is in the other camp.