неправда, явное приведение имеет такой же синтаксис
И явное приведение - это и есть каст в интерфейс:
It is not possible to access an explicit interface member implementation through its fully qualified name in a method invocation, property access, or indexer access. An explicit interface member implementation can only be accessed through an interface instance, and is in that case referenced simply by its member name.
Это из спеки