There's no doubt that FLL & MIA are still considered co-terminals. However, as I found out not too long ago (re: SFO/SJC) not all tickets allow co-terminal standby so you need check the fine print carefully to be sure your ticket qualifies.

W/that said - that's no excuse for poor customer service (I would definitely report being hung up on) and if you have reached your 'point of no return' w/AA/US - that's understandable.