An extension can only read/modify pages that it has explicitly requested to be able to read/modify in its manifest. In this case, the extension has requested to

An extension can only read/modify pages that it has explicitly requested to be able to read/modify in its manifest. In this case, the extension has requested to read/modidy "https://*/*" https://*/*" (which is necessary since Medium is often used to power other domains). It is sadly true that most extensions which do anything useful end up needing to request permission to all web sites. And that's why I mostly refuse to use extensions...