They did mention "Miyagi, a retired Army two-star general, then explained that an individual on his team sent the alert in error, even clicking through a redundancy on a computer screen intended to act as a safeguard from such a mistake."

So there was a check on it, but if the test drill looks a lot like the real thing; it easy to mix the real and test versions. If they look too differently; the training wouldn't be accurate. So there is always a tension between looking real enough, while still obviously not real.