I can confirm what others said about air drag. I've done this in the past, assuming I needed the filter on the input side. I had to use two filters, some cardboard, and plenty of duct tape to create a triangular input in order to get a decent amount of airflow.

I wish the guy on the video would take an air reading measurement near the side of the filter. I'm guessing that a lot of unfiltered air is escaping out the sides.