If they had time to fly in personnel from Afghanistan, they had plenty of time to wipe the servers and mangle the hard drives.
I have no idea if this is true, or not. However, they could easily have flown in reinforcements ahead of time, as an insurance policy.
They did not necessarily wait for a firefight to start.
In fact, it would be silly to wait for reinforcements from Afghanistan... it would all be over long before they arrived, as you intimate.