Well they could conceivably have seen or heard the missile launch, which drew their attention to the sky, then they saw the "end game". Some probably would have been looking at the aircraft or just in its direction.
The eyewitnesses I find most credible were the two Air National Guard rescue pilots, one of who initially said that the explosion was definitely an ordnance explosion, which he had seen before as a pilot in Vietnam. He later somewhat recanted that observation, IIRC.
It is possible, although they don't testify to the launch, and in fact these types of missle launches are not that loud, like an explosion.
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