No. It's not an idiomatic expression. It's a gross logical mistake. One that should be systematically pointed out and result in the public shaming of the person who used it.
I think you're right that it is a mistake. What one means to say is "I couldn't care less."
I don't think you're right that it is worth arguing about.
Fashions come and go,and it is apparently fashionable to say "I could care less" right now when you mean that you don't care very much. It's like saying "tons" of stuff, even stuff that isn't measured by weight or mass. Paper? Oh, we have tons of paper. That would be okay (even if you only have a few pounds, because it's simply an exaggeration.) Sympathy? Oh, I have tons of sympathy. That really isn't well formed, because sympathy isn't measured in units of weight. Nevertheless, it was very, very fashionable in the 90s for people to say that they had "tons" of everything. Fortunately, it went out of fashion at some point. An even more annoying 90s fashion was to put the adverb "not" after verbs, rather than immediately before the verb it modifies. "I'll go with you. Not!" "I'd like to see her again. Not!" The nerdy one on Friends started it, I think, and it caught on. That annoying turn of lexicon also went out of fashion a few years ago, thankfully. I suspect that folks saying that they have the capacity to "care less" when they really mean that they don't have the capacity to "care less" will also go away all by itself so there's no reason to go Don Quixote every time you hear it.
Choose your battles wisely, grasshopper.
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