OP missed the check-in cutoff for ORD. Thus, he was rebooked as a courtesy on the next flight. Unless he had a flexible ticket, he should thank AA for the courtesy rebook.
If OP was not sure whether the check-in deadline was T-30 or T-45, he could have called and asked.
IDB would only apply if OP were denied boarding due to an oversale. In this situation, there would be no compensation anyway.