Understand that the Hendricksons are NOT being ordered to testify. They already HAVE testified. The government just doesn't like what they said, so it is ordering them to change that testimony to something it likes better.
This assault on the rule of law is taking place even though the feds have never disputed the original returns the Hendricksons filed years ago. The feds have never issued "notices of deficiency", conducted audits, filed liens or ever done ANYTHING to assert any claim against these folks before bringing this "lawsuit" out of the blue, with its unique request that the defendants in the suit be ordered under threat of prison to declare that the government has an interest in their money which the government itself has never been willing to make in any of the ways that the law says it can and must.
In fact, the federal Department of Treasury has always agreed-- on its own formal Certificates of Assessment, and over the signatures of certifying officers-- that the Hendricksons owe no taxes for the years involved (2002 and 2003) and never did, and that their original returns are perfectly valid.