Actually, it's true. Amazon's sidewalk works in a similar way, where if the sensor is not connected to the internet, it will talk to local Echo devices like your speakers that are connected to the internet and pass the data to Amazon through your device's network.

TVs will look for open Wi-Fi networks. And failing that, they could very well do this exact same thing.

Edit: The way it works is that the echo devices contain a separate radio that works over the 868 to 915 megahertz industrial scientific and medical band, so the sensor communicates with your echo that way, and then your echo communicates it to the network as if it's coming from the echo itself, not another device. So the sensor gets connected to the network without your network realizing that it's actually a third-party device. To your network, the only thing it sees is the Echo, but to the Echo, it sees both your network, which it's connected to, and the sensor, so it's acting as a relay.