Thread networking only applies to thread compatible devices. It works alongside the Wi-Fi network, as (simpler) alternative to Wi-Fi. Many smart thread-enabled devices relay for thread signals (thread router, orange pentagon in image); Apple TV is intended as a Border Router device (black square in image), bridging to e.g. Wi-Fi so that further communication through the home network router is possible.

A Border Router is a device that is able to route packets to and from the Thread mesh. This routing happens between the Thread mesh and any other IP-bearing interfaces like Wi-Fi, Ethernet, and Cellular.
For questions about Nanoleaf bulbs, and how they name things, please contact its manufacturer.
But a thread router leader (1 per Thread mesh (/Thread mesh partition)) is an automatically assigned thread router device that “manages” the thread network (gray pentagon in image).