Bizarre ... I'm using incognito mode on Firefox, I'm not logged in, and I can read this. I don't know what's different about your setup so I can't comment, but it's definitely not asking me for a login.
Medium's login wall comes up after you have read a few articles. I assume you refresh your Incognito every little while, so you never reach that threshold.
Or you could stop clicking on Medium posts, even when their presence here on HN suggests that the submitter thought the content might be useful, valid, valuable, or interesting.
