Thread and Zigbee use the same physical layer and MAC layer (IEEE 802.15.4), so in most case it can use the same hardware and just with different firmware (if manufacturer want).
There is even a project to run both Zigbee coordinator and Thread border router on the same chip. It works! Although it causes some issue so Home Assistant no longer recommand this “multiprotocol” way, but this is a strong demonstration of the interchangeability between Zigbee and Thread.
Thread Group members is almost the same companies who found Connectivity Standards Alliance (former Zigbee Alliance). When Connected Home over IP (CHIP) project was renamd to Matter at 2019, they create Thread Group to unify the home connectivity: Only use one network layer named IPv6 and one application layer named Matter. Zigbee can’t reach this target since it use its own network layer and application layer. So they invent Thread, which still based on IEEE 802.15.4, but with IPv6 as network layer and can transport Matter on it.
There is even a project to run both Zigbee coordinator and Thread border router on the same chip. It works! Although it causes some issue so Home Assistant no longer recommand this “multiprotocol” way, but this is a strong demonstration of the interchangeability between Zigbee and Thread.