For those who aren't aware, an idiom is a group of words that have a meaning other than their literal interpretation.
And in this case, the idiom is "I couldn't care less." Most of the time it's not literally true, but it conveys the sense that the person using the idiom considers whatever it is being described as being at or near the very bottom of the list of things he cares about. So low on the list that in practical terms, he couldn't care less.
So when someone says the opposite of that ("I could care less"), it's not even a nod to the actual concept - it's just someone making sounds similar to the sounds in the idiom, without actually thinking about the words they're saying. By your thinking, "I wooden flare lens" would also be an idiom because if you mutter it badly enough it also sounds like the real idiom.