I was told that “Quoted for truth” originated in boards where users could edit and delete their posts for much longer than they can here. The purpose of it was to quote somebody who said something stupid so “the truth” would be there when they inevitably edited their post to look less stupid. More like “quoted for posterity”. The fact that it came to be used as a term of agreement is, perhaps, a fluke.
That was a joke in an early episode of Modern Family (maybe the pilot epidsode?), with Phil Dunphy explaining to the camera that he was so hip that he knew that WTF stood for “why the face”.