Aye. Completely fail to see UA's logic behind not actually inflating the vest -nor have the actor/FA blow into the tubes- in the safety video.
Maybe it's a carry-over from the old way of FAs demonstrating these procedures live in the aisle[s]?
Obviously impractical to actually deploy it then, and unhygienic to blow into the tubes.

But for the video, are they worried someone sees that and would grab the life vest and deploy it and blow into it during normal flight after they saw it properly demonstrated in the safety video?

One of life's big -albeit not that important- mysteries!