What you (and he) might be missing is knowing the difference between activating a service and delivering it.
When Sky HD came out back in the early 2000s you needed a phone line connected to the box for billing of stuff such as PayPerView and service changes.
With the widespread adoption of broadband and Internet use this is no longer a requirement.
In fact, I am surprised that Sky retained the feature. But obviously in your brother's case it is to his advantage.
The other thing you need to know is that for most* flats they have some kind of TV aerial and satellite signal distribution system in place.
When it comes to satellite, the increasing use of Sky Q means that the satellite distribution service can cater for both legacy SkyHD/Freesat and the newer wideband signal required by Q boxes and the new Arris boxes sold as Gen III Freesat. For this reason, each flat doesn't need its own dish. Think about it, when you pass blocks of flats 10 or 15 storeys high, do you see satellite dishes on the walls for each unit? No; it's centrally distributed.
I suspect that Sky already know which type of distribution system is available for a lot of the multidweller addresses across the UK. Unless your brother has moved in to a granny flat or some other one-off flat development, or he has bought a place freehold that isn't independently managed then my guess is he should just allow Sky to do their thing without worrying.
(*) Most, but not all