DNS is hierarchical so as long as the root servers point elsewhere it doesn't matter what they publish and once the root servers are updated then the new servers are the authoritive ones.
I point my computer to use a local DNS server, I ask it to resolve an ad domain... the request doesn't go out to the root servers, it gets RPZ denied at the DNS server. The root doesn't mean anything if the local DNS server claims to know the answer to your query. The same goes for a registrar's DNS server that is publishing zones it doesn't own.
What you are describing is a forwarding DNS query resolver, which can be configured to directly answer queries without forwarding them upstream in the DNS hierarchy. This is sometimes used for ad-blocking, but the archetypical example is the DNS resolvers your ISP usually provides. Their purpose is to forward your queries to the nameserver responsible for the zone you ask about.
Domain registrars, however, usually do not operate forwarding DNS resolvers. They typically only answer queries for zones that they are responsible for, and give you an NXDOMAIN response for anything else. For this reason, nobody would use a non-forwarding DNS as a resolver for their computer since it would make it near impossible to reach hosts on the Internet (unless you happen to query a record for one of the zones they provide service for).
