The thing with matX is that they changed the scope. Its now part of the USD universe. They have to work together. First they made both compatible. I expect the fruits of this mid 2023.
Current maya-usd version already supports MaterialX, you'll even see the mtlx shaders previewing in the viewport of maya. Additionally Arnold renderer also started supporting materialx from USD. So you can already start using USD in maya with materialX, and render using Arnold. But when it comes to editing/authoring…