Compared to self-hosted solutions, a medium blog had a professional look from the get-go. This was the major reason why said team member wanted Medium.
I don't think open source projects has yet caught up with Medium editor in it's image uploads, YouTube embedding, or the good looking CSS. Gutenberg and Drupal's Paragraphs has made good attempts though.
It used to partly be down to discovery too, but since then I've realised that discovery on Medium is basically non existent, and that success there seems to come down to either getting lucky with staff picks or having an existing social media following.