I'd be interested to know who their "DDoS mitigation service provider" is.
On their spare time they take down botnets: http://www.prolexic.com/knowledge-center-ddos-vulnerability-...
http://arstechnica.com/security/2012/08/ddos-take-down-manua...
Shameless plug for hackmiami, if anyone is interested in learning how its done up and close they run frequent talks/meetups locally: http://hackmiami.org/
github.map.fastly.net.
199.27.76.133
If that's indeed what they're using, the only way to tell would be to look at BGP announcements when they're actually under attack.
"A simple Hubot command can reroute our traffic to their network which can handle terabits per second. They're able to absorb the attack, filter out the malicious traffic, and forward the legitimate traffic on to us for normal processing."