Not sure but I believe that if you are using an Openthread RCP firmware and not Openthread NCP firmware then you might still need to install the Silabs Multiprotocol Add-on as it will run Silicon Labs cpcd deamon and otbr-agent on the host which you need with all types of Openthread RCP firmware. That it, your installation ith Openthread RCP firmware needs to have OpenThread drivers and core dependencies running on the host, (which are not needed if use Openthread NCP application firmware as then it runs all onboard the SoC hardware). See if can make use of the add-on → https://github.com/home-assistant/addons/tree/master/silabs-multiprotocol
Dependencies that need to run on the host computer are very different when using any type of Openthread RCP (Radio Co-Processor) firmware compare to running Openthread NCP (Network Co-Processor) firmware, as an NCP-based architecture runs practically everything on the SoC hardware while an RCP-based architecture moves most parts to the host and practically leaves the radio SoC acting as as a physical radio. See design difference here → Co-Processor Designs | OpenThread
Thus one alternative could to find Openthread NCP firmware instead. Another alternative if want to use OpenThread RCP firmware could be to manually install and configure Silicon Labs cpcd deamon and otbr-agent with all their dependencies, but that is very complicated with loads of interconnected components which is they created the addon to simplify the setup and configuration.
So if you can not get an OpenThread NCP firmware and that that instead then I suggest trying following the Matter integration documentation if using OpenThread RCP firmware → Matter (BETA) - Home Assistant
See the architecture components needed and used when using OpenThread RCP firmware →
https://github.com/home-assistant/addons/blob/7f731c837e904d6da92e062f276d61b807d58411/silabs-multiprotocol/images/architecture.png
Regardless, you posted this to Zigbee forum section where Thead and Matter is not in scope. Suggest that you instead post under configuration (or have a forum moderator move this post) → Configuration - Home Assistant Community
You might also find more information here → Chip / Matter support (including Thread) and here → Home Assistant Add-on for Silicon Labs Multiprotocol stack · zigpy/zigpy · Discussion #894 · GitHub