Since you booked this on AA miles, I would consider reaching out to AA, and letting them know that IB contacted you regarding an oversell on the IB-operated overwater segment. Maybe AA can liaise with the OneWorld partner / codeshare desk and find out if the oversell is still, infact, oversold. You could also check with AA if there are any backup options ex. ORD if IB hands you a boarding pass in Y for ORD-MAD.