Check out the Sen Proxmire “Golden Fleece” case. That went all the way to the Supreme Court and it ruled that congressmen are not immune from liability for slander.
I didn’t say they weren’t “liable.” I said, it’s very hard to prove DAMAGES. I bet you’ll find significant damages in the Proxmire case. Aside from reputation-—which we know is huge, but which courts do NOT hold as significant unless you are a celebrity who loses a lot of money-—what are the damages to the Marines?