Thanks. This is sort of what I was thinking - it seemed unlikely to me that a device would join multiple thread networks in order for multiadmin to work, but most of the videos on Matter focus on matter specifically. Perhaps I need to find go and find some deep dive videos on Thread and Border Routers.
I’m in the middle of deciding if I want to get Nest WiFi Pro, which would mean I would have 3 border routers in my home (that’s obviously not the only reason I’m thinking about it though).