Detecting motion is relatively easy, but actual positioning would require knowledge of where the WiFi router is placed within the home, and where the other static WiFi devices are. So position detection is realistically about as much as they could do without calibration.

The xfinity one is more about smart home stuff, and is opt-in (and it's a fair point to distrust that).

The reason I don't think they are tracking motion/position within your home is that there just isnt any useful way to turn that into a targeted ad, and therefore has no viable revenue path. At best, they could count how many times you go to the bathroom, maybe target a metamucil ad, but its so niche (and likely extremely noisy data) that its just not useful.

They know when your at home already, as your phone will connect to your network, and your network traffic (VPN/encrypted or not) will spike. They don't need WiFi positioning to determine that.

I'm not saying people should just trust things are okay, its more that there are much higher priorities to tackle. WiFi positioning is way down the list of realistic concerns.