I don't know if you saying "it is not AI" and then asking "how you know if it is AI?" directly afterwards is a realization of the fact you think I might be right after all but I can't give you a description of how to spot AI - I can just see it by looking at it. Some people see it, others don't.
It's still AI even though it's a moving clip. It extends to videos as well as still images. It can be as simple as a real clip that has had the original person's face replaced by another, or just completely manufactured artificially. There are hundreds and thousands of these Instagram reels these days which have been faked using someone else's face or just an artificial created face.
So I don't know how I can prove it to you, but I know I'm right. Someone said in the Instagram comments that it was fake as well if that makes any odds to you.