They offer a local instance of GitHub actually.
But hey, I guess there's the comfort in know that when it goes down it's our fault!
If you know you have a major deliverable coming up, you can choose not to touch your git server until it's at a somewhat safe moment to do so.
If you have "danger times", then any unplanned disruption in that time will hurt so much more.
The odds of GitHub going down and your local copy/instance simultaneously is very low so availability is high.
Reliability is how often GitHub or your local instance goes down.