The website has basically no CSS, so it's up to your user agent to decide on its default presentation.

Just so happens that modern user agents don't really care about bare HTML, just CSS and JavaScript, so your default presentation is optimized to 1990s hardware at 800x600.

I guess "reader mode" is slang for "what the user-agent should be doing to make documents reasonable for modern displays", so press that button in your user-agent.