From AA.com's rule 80:
1. On next AA flight in the same inventory as ticketed if available. If not available, rebook the lowest available inventory in the same cabin as ticketed.
Is the interpretation of the second sentence as "rebook the lowest available inventory in the same cabin as ticketed
on any airline" ?
Otherwise, I do not see on that link by JDiver where they explicitly say they have to reaccomodate on another airline. I can imagine the agents stating that at the gate.