Once in safety, Miranda reveals that she had witnessed the murder Joyce claimed she had seen; more precisely, she saw Garfield and Rowena drag Olga's body through the quarry garden and only later realized she had witnessed a murder.
She saw. Garfield and Rowena drag Olga's body through the quarry garden. Garfield and Rowena realized. She had witnessed a murder. She had witnessed the murder precisely. This is what Miranda reveals in safety. She had seen; more the murder. This was what Joyce claimed.