You see the whole "invasion" only through the eyes of Mel Gibson's character. There's no objective proof that any of the movie's plot action occurs anywhere but within his own mind.
Okaaay... trying to remember back to that film. Tell me if I'm wrong - I recall that Joaquin Phoenix's character was watching the TV program while Gibson was walking around (you see shot of alien ship on TV screen, voice-over of news announcer.) I'd have to see it again to be sure.
I'm not dissing your interpretation, especially since I don't recall every detail of the film. It *did* strike me as rather strange that a world-wide invasion was going on & everyone was so *calm* about it. It certainly puts an entirely different cast on that film.
So Gibson's character was simply an insane person seeing things? Unless Shyamalan himself said they weren't actually aliens, I have to assume they were.