We aren't saying that Cersei and her men didn't know what it meant. We are saying that they shouldn't have known what it meant because it was not what bells meant prior to that battle. And they had no way of being relayed that information since Jaime didn't tell anyone.
Grey Wurm re-initiated the battle after Dany began burning the civilians. I'm not sure if he knew it was going to happen or not, but he didn't act until she did. He was going to do whatever she did.