Yes it only changes the capsule rotation on Z axis (horizontal) but it also does lots of other stuff including ground checks.
By custom movement I didn’t mean writing your own system from the scratch. You basically create a new component inherting CMC. Then create a custom movement mode, copy and paste CMC default walking physics, remove whatever you don’t need, add X and Y rotation fixes, maybe even remove sub stepping (this might lower the movement quality though but improves performance considerably) and use that mode instead of the normal walking mode of CMC. Should result in way less function call overhead and also less computation. Takes considerably longer time to implement though.
You can check this play list for a very useful series on how to make custom movement from CMC: