"...saying any talks with Rybicki and Ghattas might interfere with the investigation of special counsel Robert Mueller"

The constitution prioritizes Congress getting the info it wants over prosecuting someone for criminal activity. The DOJ has it completely backwards. That is why the constitution allows Congress to call and subpoena witnesses and to compel them to testify, although the compelled testimony cannot be used in a court of law. This is because CONGRESS getting information is more important that the prosecution of crimes.