I think the point of the article is that there is potential for Wifi routers to be used to monitor people without their knowledge. An attacker could potentially

I think the point of the article is that there is potential for Wifi routers to be used to monitor people without their knowledge. An attacker could potentially penetrate your router and install malicious code on it to monitor certain things, where it would be too obvious/not viable for them to put a camera in the room. (and also the walls thing that others have mentioned)