The problem here is that although feed validates with W3 feed validator it contains some slight "errors" (see here: https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fmedium.com%2Ffeed%2F%40irrelevante). That being said, the point is in fact not the Autoblog itself but WordPress. The Autoblog plugin is using SimplePie library that's the part of WordPress for fetching feed. However, WordPress adds up some much more strict "sanitization" rules to it and simply doesn't handle that feed due to aforementioned issues. So the "flow" here is: Autoblog asks WP to fetch the feed -> WP gets the feed -> WP checks it and considers it "broken" -> WP "tells" autoblog that the feed is broken and doesn't return its content -> Autoblog can't process it further.
Example feed in question: https://medium.com/feed/@irrelevante