I’m sure lh meant exactly what he said. Is it really possible that you are unaware of the variant phrase, which has precisely the same sense?
Yes, certainly I’m quite well aware of the variant phrase and its accepted usage. I was also aware that Languagehat used the more idiomatic expression as a tease, and for this reason I questioned in jest, but not as a peever. Obviously a failed attempt at subtle humour.
Regardless, I don’t agree that the variant phrase always has “precisely” the same sense.
The Free Dictionary
(I) could(n’t) care less.
Inf. It doesn’t matter to me. (The less bears the heaviest stress in both versions. Despite the apparent contradiction, either reading of this—both the affirmative and negative—usually have the same meaning. The exception would be in a sentence where the could bears the heaviest stress: I COULD care less, [but I don’t.].) Tom: The rain is coming! The carpet will get wet! Mary: I couldn’t care less. Bill: I’m going to go in there and tell off the boss? John: I could care less.
See also: care, could, less
could(n’t) care less
[one is] unable to care at all; it does not matter at all. John couldn’t care less whether he goes to the party or not. I could care less if I live or die.
Regarding those two expressions I don’t see any confusion in the context of their customary usage. I do, however, see a potential confusion with words such as, peruse, bemuse and nonplus/nonplussed. The variant meanings for these words are antonymous and that is where the confusion might lie. If I say: I’m bemused by your statement. It can have two meanings: amused or perplexed.
I prefer, I couldn’t care less, not because it’s the correct usage, but because I’d rather be more precise to avoid any ambiguity.