You did a valuable service for other writers by shining a light on this. I've signed similar contracts for print publications, but there I'd get an upfront fee for the story. The contract then says what they can do with the story they just bought. I can't imagine having no rights on either the front or back end. Unless they figure this is in addition to Medium's contract? So, yes, they could sell your story elsewhere...but so can you. And Medium pays you for views on the platform, not them.