I did the firmware upgrade today on my Level Lock+, and it’s now a first-class citizen in both Apple Home (w/HomeKey) and also in HA. Very excited about this. Below are the steps I took after upgrading firmware and pairing to HomeKit.
After adding to HomeKit and ensuring everything worked as desired, I verified that my Apple HomePod mini and Apple TV 4K devices were detected as Thread border routers under HA > Integrations > Thread > Configure. HA detected a new integration on my network called “Matter”, and I followed the prompt and allowed it to install the Matter server add-on for HAOS. (Unsure if this step is necessary for everyone.)
I then opened the HA companion app Settings > Companion App > Debugging > Thread and clicked the “Transfer to Home Assistant” button under the Apple Keychain entry in my Thread Credentials.
Finally, in HA: Settings > Integrations > Add > Add Matter Device > Yes it’s already in use > Apple Home. The flow prompted me to open the Apple Home app, open the device, go to its Settings, scroll all the way down, and tap the “Turn On Pairing Mode” button at the bottom. This popped up a pairing number on my phone, which I entered into the HA “Add Matter Device” flow, and voila! The Level Lock now appears as a regular device under the Matter integration.
Now it’s time to deactivate all my old automations that kept the device’s state in sync between Apple Home & HA. HomeKey works with my iPhone (and presumably my wife’s Apple watch too). Couldn’t be happier right now! Sure hope this continues working well beyond just the past hour 
