If you are granted immunity, then you must testify. And, Congress has the power to grant immunity. Absent a grant of immunity, an individual has the right to assert the fifth amendment.
The prosecution may have prosecuted the case well, and the case included several landmark rulings related to the prosecution of former U.S. executive branch appointees in criminal cases ... but it was all ultimately meaningless because the immunity granted to North by Congress made any attempt to prosecute him a waste of time. LOL.