You lose quite a bit more. People keep checking if it's back online. The usual flow is disrupted. Questions might be asked about whether branches can be merged. Integration with CI might fail. Managers start asking questions why features cannot be deployed. Some of the tooling I wrote even would stop working because the API was unavailable.

Unfortunately, it's easier to down vote, than to come with a better estimate of the total cost of a 13 hour down-time of github.