Your browser window is just too wide. With such an oversized browser window, a substantial proportion of content on the web, perhaps the majority, is going to have an absurdly wide measure to the point of extreme illegibility. I highly recommend narrowing your window; as a general rule it will dramatically improve web typography.

HTML/CSS technology are really bad at giving page authors easy control over precise text layout. If they make the CSS code short and use percentages, then everything will end up too wide in large browser windows, and too narrow in small browser windows. But if they use fixed widths or maximum widths, it’s easy to screw up and break the layout in some browsers.

P.S. the author directly addresses your point in the linked page, did you read it?

“From a typographical perspective, the most appropriate method is to set box width in ems (elastic layout) as it ensures the measure is always set to the typographer’s specification. Setting box width as a percentage (liquid layout) gives the typographer approximate control over measure but also allows the reader to adjust the layout to suit his or her comfort. This website has been designed with liquid layout to afford readers this control.”