1. Stool pigeons. Which would mean Podesta brother are cooperating.
2. Disinformation.
As I understand it, under a grant of immunity for testimony, they're compelled to testify. They don't have a choice. They can't refuse the immunity either.
The only justification they could give for refusing to testify is avoiding self-incrimination. The grant of immunity removes that justification. They have to testify, whether they want to or not.