I am a native speaker and got the gist and saw the paradox, and found the phrasing a bit tortured by the triple negative. Thank you for explaining that this was a colloquialism. Now I have to go look up the etymology... And upon further inspection, this usage is actually a misnegation.

"It is a veiled insult: an ironic form of insult delivery which is misinterpreted as flattery to the buffoon who is targeted by it, much to the entertainment of anyone else within earshot who understands the true meaning."