Well....I can understand how "creatures from the sky" could be misconstrued as Angels...simply because back in those days alien visitations were unheard of. People didn't understand, or know that they were not alone in the universe. I'm presuming that back in those days...Heaven was basically "in the sky" and God and his Angels resided in Heaven, so anyone coming from the sky would be considered an Angel, or messenger from God. Very simple logic on that. This is not to say that the people of those days were dumb. This just means that they had no understanding of alien visitations and would naturally presume that creatures from the sky were Heaven sent.
There are many tales among the Native Americans about "Sky Warriors" and visitations by them. The Navajo People have several of those stories. They tell of Sky Warriors coming on "Flaming Horses", or "Horses of Fire" and coming from the sky. Here again...could these Flaiming Horses, or Horses of Fire be UFO's coming in through the atmosphere? As we all know...our known spacecraft, when reentering, get rather toasty on the outside when they hit the atmosphere and I would presume it would be the same for a UFO.
I am thinking that the Biblical story could possibly be based on an actual event of alien visitations.