Actually, that's not what Medium's own rules about plagiarism say.

"Medium defines plagiarism as any act of taking the words and/or ideas of others and presenting them as original or without proper acknowledgement or permission. This includes directly copying ideas and text (whether entire articles, paragraphs, or sentences), as well as paraphrasing and slight re-writes (often known as "mosaic plagiarism"), and failure to cite sources.

Medium requires that the sources of quoted or paraphrased content potentially included under Fair Use be properly cited within posts. Proper citations of sources in any of the major systems of citations are welcome, and can be added using numbered superscript text formatting, or other appropriate methods."

For whatever reason, they just don't care about rampant plagiarism on their site.