The person running the scan has to know how to interpret the results. There's a % score assigned by the program. It will also highlight the potentially offending passages so it's easy to check if they've been attributed. But by the way, attribution means not plagiarized, technically it doesn't mean free to use. I've never seen a Medium writer complain when I quote them but it would be within their rights to do so.

Song lyrics are a big no-no if quoting more than a few lines (your usage is safe). And yes, the plagiarism detectors will flag passages of the writers that they may have copied from themselves, but that's why the policy was not to accuse the writer but to ask them to explain the result.