That part is in the Constitution (Article I, section 6: "for any Speech or Debate in either House, they shall not be questioned in any other Place").
Indeed it is. I thought it was an interesting but tangential and irrelevant point to this suit. The wrongful death part will be tossed, but Hillary will have to pay some money to get a good defense for the defmation. The judge will toss it. No reasonable judge would find her liable for anything.