I would like to first clarify about interface views in Denodo. They are basically a special type of view to do top-down design which consists of definition of fields and a reference to another view. Where you can first define the fields of the interface and at a later stage you will be able to associate to a "implementation view" of the interface.
Therefore a query that involves a interface is always delegated to the implementation view. So when you would like to improve the performance of an interface view then you need to neccesiarly generate the statistics for the implementation view.
Hope this helps!