It sounds like the state's position is that the MEs only looked at the body after death, whereas Dr Perper looked at the bigger picture. That would make sense to the jury I think - at the time no one was pushing the idea of murder, so the MEs did what made sense at the time. As more info came forward, it needed another look from a different angle.
Is that the general idea of what the state is doing? If so, I think it's a smart move not to argue that the MEs were incompetent, leading to the need for Dr Perper to take a look.