I’ve got some Nanoleaf Thread bulbs, with the xxx-xx-xxx pairing codes.
My Google Hub is set as Preferred Network in the Thread integration.
When I add the bulbs to the Nanoleaf app they become visible in HA but after pressing Add and entering the 8 digit code it says “This accessory is already paired to another device. Please reset the accessory and try again”
I assumed this was the Nanoleaf app but after deleting it I get the same result.
I was having the same problem last week. what worked for me was setting up the bulbs next to my HA server. I am running as 2012 mac mini on bare metal. Before i was using my cell phone from the other side of my house and I was only able to pair 1 of 6 of my old homekit bulbs. Moved setup to my server and added the rest in a flash.
I don’t use Homekit, so not as familiar with it, but I know of this thread that may help out. Note in particular, how the Eve Motion Blind was added to HA’s Homekit similar to what you are trying to do (via Google Nest Hub).