I think he has copped the fifth, after having been given immunity.
Which he cannot do, it puts him in contempt of Court.
You may be conflating two separate investigations.
Bryan Paglioano, the technician who installed Hillary's private serve, reportedly negotiated an immunity deal with the FBI and is 'singing" to their interrogators.
Separately, he has refused to testify to a Congressional committee -- which he is free to do, since the immunity deal does't extend to them.
Two different situations. Pagliano is refusing to testify in front of Congress, but he is testifying to the FBI, having been granted immunity by the latter.