As I understand it, and you may correct me on this, Medium is supposed to be a publishing and networking platform where everybody can come to express their ideas and tell their stories, a place open to a diverse range of opinions. If the management of this platform have their own opinions, they may (and do) publish articles accordingly. They have not, until now, overtly put forth a political stance that is front and centre in their mobile app, a feed that is devoted to anti-Trump articles. (I’m actually surprised they haven’t done this on the Medium website, as well.)
You and I don’t have to be neutral. (In fact, most of my articles are highly opinionated; they often lead to fierce debate.) We are users of the Medium platform. Management, however, should be dispassionate, at least across the board. Ev Williams or any of the other staff may express their political opinions in the same way as you and I do, but not to shove it in our faces.