OK let's see if we got this clear. Your scenario is:
a) SQLServer in the backend
b) Denodo as the middle tier. SQLServer conencted to Denodo as a data source of type JDBC
c) .NET application as the client application, connected to Denodo
In a scenario like this, the vendor of the sources connected to Denodo is irrelevant once the base views are created in Denodo.The fact that you have a SQLServer in a) doesnt matter. You could have Oracle, DB2, flat files, web services, etc, or any combination of the above. Denodo's engine takes care of that.
But from the client application perspective, your .NET application is connected to Denodo. And the conenction to Denodo is done via the npsql data provider, connected to the Denodo server. This doesn't mean that you are conencted to PostgreSQL. You are conencted to Denodo, we just use the Postgres drivers.
In execution, your data will come from whatever sources are conencted to Denodo, in your case SQL Server. But that's transparent to the client application.