Why would you use a blockchain? There's already a central authority - the DC city government. They should just host their own git instance.
So the activity is likely happening on the larger and more exposed GH service, while internally there is a daily copy bring backed up that isn't (as) exposed to hackers.
Something life that would be a great way to displace risk.
Doesn't seem likely.
Seriously, I'd imagine they treat it as “someone else's problem” — like backups.
If GitHub gets hacked, I imagine their plan is “be affronted, and sue”. When GitHub's lawyers point out that the terms of service don't guarantee anything, the plan says “be even more publicly affronted that GitHub refuses to take responsibility, while taking no responsibility”.