If their plan is to get online via the second location, it's likely that said location has a much beefier upstream or built-in filtering, allowing them to absorb these amounts of traffic without being null-routed.

More likely, though, they're restoring the service to a fresh IP range and put the servers behind some kind of DDOS-protection or, alternatively, they simply choose to do the switch now as they need to do a full restore anyway and it's not related to the DDOS mitigation.