That is a very strange take. The phrase isn't American and has no negative connotation. It has nothing to do with "extrajudicial punishment". It simply refers to the (obvious) fact that what separates societies from anarchy is the "thin blue line" of law enforcement.
Rowan Atkinson had a sitcom set in a London police station in the 90s called "The Thin Blue Line". Are you under the impression he was dogwhistling about extrajudicial violence?
Rowan Atkinson had a sitcom set in a London police station in the 90s called "The Thin Blue Line". Are you under the impression he was dogwhistling about extrajudicial violence?