Matter was created by the Connectivity Standards Alliance (CSA), formerly known as the Zigbee Alliance! Basically, Matter is the next generation Zigbee. Thread as a protocol predates Matter, and it is just one of the supported transports, together with Wifi and Ethernet (for now).

Edit: One thing Matter adds that was not in Zigbee is Bluetooth provisioning, letting you use your phone to add a device to your network without QR codes or numbers to type in.

Also fun fact; Homeassistant is part of the CSA and apparently, Google, Apple and others use HA for testing!