I saw Github's code as a consultants years ago, and I always thought it was crazy that they would ship the whole thing to us. But then I thought, how many employees do they have? Probably enough that security should not rely on the secrecy of the code anymore.
I think the main concern would be the intellectual property around the large-scale distributed system which is storing billions of lines of code in a single system. Implementing git object storage is still hard (e.g. https://en.wikipedia.org/wiki/Virtual_File_System_for_Git).
Though, that's probably not what they use in the enterprise edition, so maybe they aren't as worried.
