I don't think the nameserver (or the upstream NS) is the target of the attack, but the (spoofed) IP which appears to be sending the queries:

* Attacker sends (small) query to a lot of resolvers, spoofing the source adress to be the IP of the target

* Each NS replies with a (large) response, thus flooding the target with a lot of data

As long as there is no rate limiting in the nameservers used for the attack, this would work regardless of whether the answer is authoritative or not.