This was worded misleadingly. This is indeed a DDoS: code has been injected to load the Github pages in the background using XHR without the user's knowledge. The host page itself is not redirected (or visibly affected in any way[1]).

Furthermore, only people outside of China are affected by this -- Chinese citizens don't have this code injected.

[1]: Actually there is a mistake in the injected code that causes the result of the XHR request to be interpreted as JavaScript, and then executed. Hence GitHub has tried to mitigate the attack by replying 'alert("WARNING: malicious javascript detected on this domain")' to notify the user that this is happening.