The point is probably that if someone is observed or heard in the room, the applicant knows it would invalidate the interview and so won't risk it.
I'm sure it's nothing like the proctoring requirements when DD did her finals from home in 202, at least! Not only did the camera have to be on continually on the laptop, but she had to have a second video feed (her phone)- scanned round the room before starting and then set up so as to show where she was working for the duration.