The post hints at the history of DVCS within Google Code and why they have added support.
Since our original announcement of Mercurial support, Git has grown significantly more popular and user-friendly, and on the technical side, it has added an efficient “smart” HTTP protocol that fits with Google’s HTTP-based infrastructure. (Note that this feature is only available in version 1.6.6 and later.)
The blog post goes on to briefly describe some of the technical aspects of the service including a note that states
"to fit with our existing Python-based system, our Git server implementation is powered in part by Dulwich" which is a pure-Python implementation of the Git file formats and protocols.
I, for one, welcome the new option and will be curious to see if it gains any traction against GitHub.