smart home companies have been deploying the thread hardware for years before anyone started using it

Also worth mentioning that many modern Zigbee radios can also be Thread thread radios using different firmware. There are even multi-PAN radios that can do Zigbee and Thread at the same time. Some smarthome hubs use multi-PAN (e.g. Homey Pro), but it's generally recommended against now because of lower reliability.

The same applies to devices, e.g. some of the new IKEA devices work over Thread or Zigbee (Zigbee pairing is triggered using a non-documented sequence, presumably they added support for TouchLink). Or e.g. the Aqara FP300, which can be flashed with Thread + Matter or Zigbee firmware. It works because the same radio can be used for both protocols.