This is just a quirk of DNS: The nameserver name has an A record served by both the TLD’s DNS servers (the glue record, which is how everyone finds your DNS server in the first place) and your own DNS server. In practice your domains may work with only the one at the TLD, but it’s not something you want to try. The records that your DNS server serves are considered authoritative, and a missing record could override the information gotten from the TLD in weird cases.
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