That is the lie they are trying to repeat until it becomes true.
The US Supreme Court has already ruled on this sort of immunity issue, stating:
(a) By its terms, 1983 "creates a species of tort liability that on its face admits of no immunities." Imbler v. Pachtman, 424 U.S. 409, 417 . Its language is absolute and unqualified, and no mention is made of any privileges, immunities, or defenses that may be asserted. Rather, the statute imposes liability upon "every person" (held in Monell v. New York City Dept. of Social Services, 436 U.S. 658 , to encompass municipal corporations) who, under color of state law or custom, "subjects, or causes to be subjected, any citizen of the United States . . . to the deprivation of any rights, privileges, or immunities secured by the Constitution and laws." And this expansive sweep of 1983's language is confirmed by its legislative history. Pp. 635-636. [445 U.S. 622, 623]
[…]
(d) Rejection of a construction of 1983 that would accord municipalities a qualified immunity for their good-faith constitutional violations is compelled both by the purpose of 1983 to provide protection to those persons wronged by the abuse of governmental authority and to deter future constitutional violations, and by considerations of public policy. In view of the qualified immunity enjoyed by most government officials, many victims of municipal malfeasance would be left remediless if the city were also allowed to assert a good-faith defense. The concerns that justified decisions conferring qualified immunities on various government officials - the injustice, particularly in the absence of bad faith, of subjecting the official to liability, and the danger that the threat of such liability would deter the official's willingness to execute his office effectively - are less compelling, if not wholly inapplicable, when the liability of the municipal entity is at issue. Pp. 650-656.
The same reasoning applies to US CODE Chapter 18, Sections 241 & 242; which are federal felonies for "Deprivation of rights under color of law" and "conspiracy against rights."
While I agree with you, not so many DAs are willing to file charges against police officers, even those who murder. I am also of the opinion that prosecutions under 18 § 241 and 242 are tragically rare and should be pursued in much greater number. The bigger lie is that somehow the separation of powers results in a meaningful check on government power. In fact, agents from all three branches are “playing on the same team” and tend in most cases to support one another, against the citizenry, rather than going against one another, in support of the citizenry.