alert("WARNING: malicious javascript detected on this domain")
when you're trying to reach: https://github.com/greatfire/ or https://github.com/cn-nytimes/
That's why other people have started to notice it (as the article describes).Timeline of the attack looks something like this:
1. The Chinese firewall hijacks requests to http://hm.baidu.com/h.js and sends back a script that attacks GH instead.
2. Github notices that a huge amount of people are trying to reach https://github.com/greatfire/ and https://github.com/cn-nytimes/
3. GH figures out what's happening and starts replying with the alert javascript snippet.
4. Users are now getting noticed by the alert every time their browser runs the hijacked javascript.
5. The person that wrote this article writes this article after investigating what the alert message is about.