Every page on that site returns a 200 status code, and the same content as the homepage.
This means that http://www.highcharts.com/license (linked to from GitHub) is currently failing silently - you get content, but it's not the licensing information.
I've been seeing this as a default behavior for some static site hosts. If you don't supply them with an error.html, they redirect 404s back to the homepage by default - but with a 302, not a 404 + meta redirect.
I'd much prefer a generic error page supplied by the host, even if it doesn't match the brand of the rest of my site - 404s are a pretty important piece of the internet.
This means that http://www.highcharts.com/license (linked to from GitHub) is currently failing silently - you get content, but it's not the licensing information.
Note that http://www.highcharts.com/licenseoneutahenuo returns the exact same content again.
404s are useful! Don't disable them.