It isn’t false, just not complete. Sure, someone may file the complaint in U.S. District Court. But the complaint would be promptly dismissed by the court due to sovereign immunity. So more accurately one should say that a plaintiff cannot successfullly sue the federal government for defamation. While the Federal Tort Claims Act (FTCA) allows persons to sue the United States in court for many torts that employees of the federal government may commit in the course of their official duties, defamation is not one of them. Defamation claims against the federal government (and lawsuits against federal employees acting within the cope of their employment are suits against the federal government) are expressly prohibited by 28 U.S.C. § 2680(h).
A Bivens claim refers to claims in which the government is being sued for a violation of a constitutional right. In Bivens v. Six Unknown Agents, the court held that it inferred a cause of action against the federal government for violations of a constitutional right even though federal statute did not expressly provide for it. The Court emphasized the particular importance of constitutional rights in making this decision. It is thus a narrow exception to the usual rule of sovereign immunity that says that the federal government may only be sued where Congress has by statute waived the government’s immunity. The problem for using Bivens here is that there is no constitutional right against being defamed. That being the case a Bivens action would not get a plaintiff around the restriction against defamation claims in 28 U.S.C. § 2680(h).

