I think what they meant is that the backup camera screen is embedded in the review mirror. I’ve seen those that it’s kinda like a first surface mirror and you can see the screen only when it is turned on. Otherwise it’s mostly invisible.
Sorry for the Amazon link, but I haven't had enough coffee to Google better: https://www.amazon.com/AUTO-VOX-T2-Monitor-Waterproof-Revers...
The videos in the reviews do a better job than the actual product videos of showing what is going on.
I feel that a good backup camera system gives you a better view of a wider area than just using the standard rearview mirror or than turning around and looking out the back of the car.