I personally do not think that it is the CMs fault who gave the closing announcement, however when these things happy lower level management want to seem "proactive" so all yesterday CMs and CPs were being interviewed and questioned, and interrogated about the pool closing procedures, who gave the closing announcement, what the person's volume level was etc.
If making sure everyone at the pool can hear the closing announcement Disney should have provided these lifeguards with Bull Horns, or a PA system. Instead of suspending a College Program kid for not shouting loud enough.