I couldn't easily figure out just who this guy was (i.e. handle) but it seems somewhat unusual that he would've performed the hack alone. Especially given the malware deployed[1] and information shared by the victims [2].
>What's the benefit?
Tons, having personally tried the same thing in the past :^) Being able to serve backdoored copies of the kernel to targeted users would be all kinds of useful, and you'd be in an unique position to target the kernel developers themselves, allowing you to actually insert a backdoor into the kernel.
[1] Phalanx is by far one of the most advanced pieces of publicly analysed linux malware https://volatility-labs.blogspot.de/2012/10/phalanx-2-reveal...
[2] Kernel.org folks claimed that they weren't the only target, which is to be expected (people with no prior hacking experience rarely decide to acquire/produce somewhat advanced linux malware and install it on kernel.org)
[2] ctrl+f credential-stealing https://lwn.net/Articles/464233/