Building on HeyJude's answer, slightly less tedious than scrolling with the mouse-view.
Open the Developer Tools Console.
Spam these lines a few times to scroll to the bottom of the page and trigger more articles to be loaded on the page:
headings = document.querySelectorAll('.EntryList__heading'); headings[headings.length-1].scrollIntoView();
Then, once you've loaded a month's worth of articles:
document. querySelectorAll('.EntryMarkAsReadButton'). forEach(buttonElement => { buttonElement.click() })