Great advice. I never write in the Medium editor. I write everything for Medium in a Scrinever file called Medium (clever, huh?) Then I copy and paste into a new Medium story. Every story I've ever written on Medium is in that Scrivener file, and that file is automatically backed up to Dropbox (off site) and Time Machine (local.)
It looks like Medium plans to turn into a blogging platform, so we'll see what happens.