I think he (or she) is saying that if it had been a vehicle piloted by extra terrestrials; the fact that it was destroyed via explosion/collision proves it was vunerable to something at least. And if the millitary had found said UFO (or even shot it down), then they would have mobilized to defend against other possible UFO's. But since they didn't do so, it must not have been a UFO.

Hope I translated that correctly.

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I'm like one of those idiot savants...well, except for the savant part.