Stopping coders from deploying stuff due to a risk of an unspecified "something" going bad makes for frustrated employees. By that time you might as well give them the days off as they'll have little to strive toward. You don't need to freeze all code deployments and other things that have little risk.

Also, they most likely scheduled this at this time due to the lower traffic, probably the lowest of the year for them. While half of Github was probably enjoying their families the other half was planning for this for a long time. Given the size of the operation I don't think anyone took it lightly.