I don't believe that congress can give immunity from a constitutional point of view. Justice would have to concur with this for it to be enforcable. Seperation of powers would allow justice to ignore any immunity in order to prosecute. (Same same for a serving member of the military if that would come up with regard to military justice.) The congress could set aside a treason conviction by a 2/3s vote to clear this person for public service but this is moot if a hanging can be arranged.
its obvious that whomever this person is, they are trying to buy cover by declaring themselves a whistleblower to avoid an investigation and possible prosecution. the Dems will be going to bat for this person big time. Hopefully the FBI knows who this is, and is obtaining all their cellphone, telephone and email records to see if the NYT and/or the DNC is on the other end of any of it.