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.

This service and similar ones live on because a lot of routers support setting your dynamic IP to a dns name by using these services. The services are incredibly simple but finding a service that your server actually supports is a gamble.

Set your system’s resolver to the iana roots and see what happens