Hello Moon. Yes, you're right. Your understanding of that use of but is correct, and I'm not surprised the form is not familiar to you because it's almost entirely gone out of use - I can speak only for British English.
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Thomas TompionNo head injury is too trivial to be ignored.I don't think I'd really call that a 'triple negative'.
fivejedjonI don't think I'd really call that a 'triple negative'.I think the people who do so reason as follows: