My understanding is that a DNS Amplification attack involves sending a DNS resolver a bunch of requests with a spoofed sender address, which is the victim's. The DNS responds to this IP address with a whole ton of info. Where is the distributed nature that makes this DDoS and not DoS? I could see it being DDoS if the attacker used multiple DNS resolvers simultaneously, but the linked article doesn't make it sound like that's the case.
From the resource you cite:
A misconfigured Domain Name System (DNS) server can be exploited to participate in a distributed denial of service (DDoS) attack.
From my understanding this means, that the distributed nature of the attack is only given if multiple servers are involved. "participate" is not really used when the affected server is the only one involved in the attack.