If they're buying stuff then it's not a DDOS. If it's millions of garbage IoT devices hitting your servers and running whatever the most expensive (CPU/Memory/IO wise) queries constantly then yes, that's a DDOS.
The DDOS extortion guys are getting really good at their job. They even broke Akamai a few weeks ago. It's a spiral where they extort money and use it to improve their infrastructure to be able to extort bigger and bigger players. I have zero doubt that Alibabba's servers were already straining under the load of that many shoppers. Adding a DDOS on top of that would be catastrophic.
The DDOS extortion guys are getting really good at their job. They even broke Akamai a few weeks ago. It's a spiral where they extort money and use it to improve their infrastructure to be able to extort bigger and bigger players. I have zero doubt that Alibabba's servers were already straining under the load of that many shoppers. Adding a DDOS on top of that would be catastrophic.