This particular case relied on botnets being able to spoof their their source IP to match github's so the memcached responses would go to github.

Google 'DRDoS attacks' to learn more. They are responsible for most of the largest volume attacks IIUC.

I understand the attack, I dont understand how it could be mitigated using the same technique.

There are many scenarios where you send of traffic to another datacenter using a GRE tunnel or EVPN.