When used in that way, it is being used wrongly, and has somehow become common in the US. Just as when they say "I could care less" but mean the opposite. It's not that the phrase is strange, it's the diction of the speaker that is strange. In countries that are not the US, people say "I couldn’t care less" and mean it to be understood the way it was spoken.
“all but” - I hate that expression!
When used in that way, it is being used wrongly, and has somehow become common in the US.
Just as when they say "I could care less" but mean the opposite. It's not that the phrase is strange, it's the diction of the speaker that is strange. In countries that are not the US, people say "I couldn’t care less" and mean it to be understood the way it was spoken.