Watch the video the article is based on - not connecting to the internet doesn't keep you safe in many cases: https://www.youtube.com/watch?v=6IFVTcM28KA
Also, wouldn't this be trivial to test by just setting up your own network and seeing if the TV connects?
They can just start a capture, record to RAM / storage, and retrieve it later when it is reconnected to the network.
Otherwise if it's disconnected, it is still hard to exfiltrate the data somewhere. Maybe they could get creative via Bluetooth, but then you would need a cooperating device in proximity of the device.