>In order for clients to find your nameserver to figure out what
>NS1.example.com resolves to,
>it first needs to be able to find a nameserver for Example.com,
>which is NS1.example.com.
>This is what is circular without a Hint in the Additional section of
>the DNS reply from the parent nameserver.
That's true, but there's also cross-domain name servers. I have
domains in .org with nameservers in .com. The org. registry won't
publish the NS records if my registrar hasn't told the .com registry
to push the name server records out to other TLDs.
Those are two different kinds of glue, but in practice you need both.
You could argue that .org doesn't need to publish cross-domain glue,
and you would be right, but you still won't get your .org NS published
if they don't have the glue.