LBR20 + RBS20 adding another compatible satellite

I realize this is an old dead dead thread or conversation, but I found something very interesting with my Orbi LBR20 when I first installed it.
A couple of years ago I pulled an old Netgear LTE/router out of storage. The LTE part had died, or maybe it was only 3G, I can't recall the reason I retired it originally, but I had set it up to be a simple WiFi access point. I connected it to the Ethernet RJ45 switch I had down there, and disabled all routing, chose setup as access point so that I could use WiFi devices down there on the same subnet as upstairs. Not only that but it gave me another RJ45 switched connection for my TV down there. 
When I got my new Orbi this week and set it up, I changed the IP address scheme to match my existing network and found something odd right away - it said it had backhaul. Now how in the world was that?  The old Netgear R6220 had connected, received an IP from Orbi and Orbi decided it was a satellite, apparently. It was so simple, so seemless and it just worked. In fact the FireStick connected to that old R6220 I was using as a basic access point was the very first thing to show up in Orbi's connected devices list. 
Wish I had another old R6220 to use in my shop for an access point out there!