Not true. First, in most states and in federal court a witness can invoke the 5th in a civil proceeding. However, when a witness invokes the 5th in a civil proceeding, the jury or judge may draw negative inferences from the witness' silence and presume that the witness is guilty.
Second, a witness may invoke the 5th when testifying before a congressional committee and the committee may grant the witness immunity with the consent of 2/3rd of the committee membership and approval of the federal district court.
I hope somebody remembers to tell the witnesses that by fully testifying in the open about the misdeeds of others they will win their cases at the MSPB. That will pay their legal fees, so lawyer up!