BARB is talking about a potential audience. In my building, for example, it would consider that there are 12 households with Terrestrial TV as potential audience. There's an aerial on the roof, and in 2018 when I last checked, the routine job to fetch the TV listings out of the data stream was still working on a PC I had connected to the aerial socket. That job may still be working but I haven't checked - like most people in this building, and your comment's parent, I don't actually watch live TV so I don't need the listings data any more if it's still being collected.

So that's 12 households you're counting as "TV viewership" that don't watch live TV. They could watch any hypothetical TV show or advertisement, but they in fact will not. They would not disagree with your parent poster.

My mother (retired) is the only person I know who watches live TV. My Facebook is full of people chatting about TV shows (Sabrina, Doctor Who, etc.) but none of them watch it live. Some of them have paid subscriptions to various sources of TV, and many of the same people also have "pirate" hookups to get TV that isn't available in a timely fashion, as described many times they aren't unwilling to pay they're unwilling to be inconvenienced.