I think that the point there is that most users (and especially for RPC-ish applications, which is what most of the existing XSD documents are about) do not need a formal schema but the schema is only implied by the implementation. The issue there is that even if you have detailed formal schema and generate the interface code from that, there is still some ad-hoc contract between the generated interface code and rest of this application, and details of this ad-hoc contract will invariably leak through the interface code.

Designing formats for long term storage of some kind of data is another thing and there formal schemas make sense. But that is not something that most users (meaning developers) do frequently or often even ever.