Technically works but not well enough to move past “experimental”
> This experimental firmware has been available since December 2022. Through extensive testing, we have found that although it works in some circumstances, it has technical limitations that lead to a worse user experience. We now do not recommend using this firmware, and it will be experimental for the foreseeable future. Instead, we will focus on making sure the dedicated Zigbee and Thread firmwares for Home Assistant Connect ZBT-1 deliver the best experience to users.
To clarify this: Thread is not building on top of Zigbee, they are both independently built on 802.15.4