Well thank you for those links. From what they say it does seem that all the disks use the same channels.
I don't think I know enough yet about the system, or have been persuaded sufficiently, to be confident enough to invest in the system.
Having all access points on the same channel makes it much easier for the network to manage client connections as the radios can "see" traffic from all devices on the network during normal operation, the radio can only operate in one channel at a time. The same goes for clients making decisions about roaming. They can see all of the APs are on the same channel and belong to the same network.
BT Whole Home monitors the Wi-Fi usage and makes decisions also based on the load of the channel so can move devices away from the band with interference. I think neighbour interference is only really an issue at 2.4GHz - 5GHz does not penetrate brick walls as well and uses 80MHz channels instead of 20MHz which gives nearly 4x as much bandwidth so does not become overloaded very quickly. 2.4GHz is more for legacy devices that do not support 5GHz or just offers a little extra range outside the house.
"Mesh" systems are still fairly new to the home network market. I think over the next few years they will become even smarter and will function better using different channels.
I took the plunge last week and purchased the sytem. I can honestly say it has been the best decision I have ever made in terms of broadband equipment. My house is pretty big with four bedrooms, two bathrooms and spread over 3 floors. It is also very wide so wifi signals struggle to cover it.
But the BT Whole Home mesh system was painless to install (took about 10 minutes) and is providing flawless wifi to every part of my house and to my front and rear gardens. No problems with signal interference from the discs or from neighbours, best speeds I have ever had, and seamless tarnsfers between discs as I move around the house.