Or maybe he could care less, but doesn't even bother to care less because caring less would exert effort and he doesn't care enough to exert any effort.
"Could care less" used as a snarky response makes sense, as in, "I could care less, but I don't want to put in the effort." Using that phrase without a sarcastic intonation is still incorrect.
Could care less meaning couldn't care less. It's the same thing as how literally has come to be used with the meaning of figuratively. If you look up "could care less" in the OED you'll find it lists it under American English with the meaning "could not care less."
