Nothing you mention is related to this article and neither Rust or C# solve the expression problem.
The expression problem is about being able to extend both data types (new cases) and operations (new functions) without modifying existing code while preserving static type safety.
C#'s extension methods can't be virtual, so you can't use them to actually add any new operations which can be dispatched on. They're just syntactic sugar for adding static methods which in non-OOP languages can be accomplished by declaring a free function.
Rust traits let you add new types (just impl the Trait), but it doesn't let you add new functions, so it doesn't do anything to solve the problem.
The expression problem is about being able to extend both data types (new cases) and operations (new functions) without modifying existing code while preserving static type safety.
C#'s extension methods can't be virtual, so you can't use them to actually add any new operations which can be dispatched on. They're just syntactic sugar for adding static methods which in non-OOP languages can be accomplished by declaring a free function.
Rust traits let you add new types (just impl the Trait), but it doesn't let you add new functions, so it doesn't do anything to solve the problem.