I would say your opinion is complete nonsense. Believe in God or not, in the wolrd of It's A Wonderful Life we are shown God is real, Angels are real and they intervene in the World when necessary.
Why would the Angels, Clarence and his superiors, speak among themselves as angels if they were aliens?
Because aliens are REAL and angels are NOT.
Show us the bodies! Belief in aliens is just another religion.
We're not shown anything that opposes what i say in the movie. We see a few blinky lights and off screen voices.
That's it. Who's not to say they aren't aliens communicating? Maybe the aliens call themselves angels.
But they are aliens. And you're free to believe in god or whatever else but the movie is a SciFi movie with alien called Clarence and a parallel universe being shown.
There is no use arguing with someone who deludes themselves.
Depending on your belief system, this could be classified as a fantasy. But it is certainly not SF.
You sound like Mr. Terrific in DC comics who had met an actual angel and convinced himself the angel must not actually be an angel. If it doesn't meet my opinion on what exists, it must be something else.