They can be. Because of course there are poorly made commercial units that are only quiet because they have low CADR. But you’ll never be able to get a DIY box as quiet as a well made unit, unless you basically engineer your own air purifier device, use blower motors, etc., but that’s not a Corsi-Rosenthal Box.
In the spirit of dumbing down the plot to make sure viewers don’t turn away, they end up making totally nonsensical (if not misleading) content.