How can you extend UnityEngine.ProBuilder.MeshOperations when it's immutable and not public?

In the ProBuilder Scripting API documentation page: https://docs.unity3d.com/Packages/com.unity.probuilder@5.0/manual/api.html

it says you can extend the ProBuilder MeshOperations class but it isn't public.

If you try to set it to public so you can inherit from it or reference in your own code, you get a warning and the code reverts to private again:

The package cache was invalidated and rebuilt because the following immutable asset(s) were unexpectedly altered: Packages/com.unity.probuilder/Runtime/MeshOperations/Subdivision.cs

How can you extend or make use of the ProBuilder classes in your own code if the classes can't be accessed due to protection level?