It means that for those zones, they explicitly put the IPs of the edge servers in their resolver (Unbound) configuration, so that lookups of names in those zones don't have to go the root servers and then the TLD (like .com) servers, only to find out that the authority (the edge servers in their design) are in the next rack. Instead they will go directly to those edges. This gives them "Addtionally, public zones are completely resolvable within our network without needing to communicate with our external providers. This means any service that needs to look up api.github.com can do so without needing to rely on external network connectivity."