there is a source of light - perhaps a window - opposite the wall where the shadow first appears. someone is walking slowly in front of it. you can also detect a bit of shadow in the foreground of the video as the person is walking
then as it gets to the doorway the shadow is enlongated and appears on the further wall in the next room
The truth is incontrovertible. Malice may attack it and ignorance may deride it, but in the end, there it is.