You're conflating two different concepts: DNS hosting, and DNS resolving.

This service seems to offer hosting services, i.e. publishing your domain's records in the DNS in DNS authoritative servers.

A DNS resolution service looks up and maintains large caches of DNS data on your behalf, optimizing DNS lookups by not requiring your own device to recursively query many different DNS servers. These lookups start by querying the DNS root zone, which is published by the DNS's authoritative servers which are listed in the root hints file.