I think their point is that a simple infra red sensor like outdoor motion detector lights will do what you need. There's zero need for a camera to do this. Massive, massive overkill.

There's a fairly large and thriving market in zigbee and/or zwave based "human presence" sensors which can serve dual duty as motion detection via infrared and also using very basic radar, for both turning on/off lights and also to provide data to a DIY security or automation system in Home Assistant.

There have been significant price drops in small modules for short range, rudimentary V-band (60 GHz) radar to do this sort of thing, which companies can built into their consumer-priced product.

I think some of the better ones are a little bit too power hungry to be entirely wireless on battery, and have a USB-C port in their rear, you're meant to plug it in persistently to a small USB charger and tuck the cable away somewhere.

Hell, 'SpaceSense' and other similar tech enables it with existing wifi radios and a software update.