Okay, this sounds as if I'm splitting a hair, but it's a very, very important hair.
I understand that the hospital that the ER was in, was in-network. That does not necessarily mean that the ER itself was in-network.
We need to know if the ER, not just the hospital, but the ER itself, was in-network. I have recently been seeing, for at least one major, national insurance carrier, ER's listed separately from the rest of the hospital.
You may need to get that information from the insurance provider.