the GOV top-level domain

A has not been the authoritative source for .GOV for a while.

Check WHOIS, it lists as a name server.

Government (US Federal) top-level domain (GOV-DOM)
   General Services Administration
   Center for Electronic Messaging Technologies
   1800 F Street, NW
   Washington, DC 20405

   Domain Name: GOV

   Administrative Contact, Technical Contact, Zone Contact:
      Borgoyne, Gary (GB260) gary.borgoyne@FED.GOV
      202-501-6824 202-501-1337

   Record last updated on 02-Oct-97.
   Record created on 01-Jan-85.
   Database last updated on 21-Sep-98 04:08:47 EDT.

   Domain servers in listed order:



To see this host record with registered users, repeat the command with
a star ('*') before the name; or, use '%' to show JUST the registered users.

The InterNIC Registration Services database contains ONLY
non-military and non-US Government Domains and contacts.
Other associated whois servers:
   American Registry for Internet Numbers -
   European IP Address Allocations -
   Asia Pacific IP Address Allocations -
   US Military -
   US Government -

Besides even if it wasn't also server the domain, the root-servers
should still return the NS records of the servers that are
servering top-level domains.

See for example the DIG for the SG top level domain.

# dig @ sg. ns
; <<>> DiG 2.2 <<>> @ sg. ns
; (1 server found)
;; res options: init recurs defnam dnsrch
;; got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 10
;; flags: qr rd; Ques: 1, Ans: 5, Auth: 0, Addit: 5
;; sg, type = NS, class = IN

sg. 172800 NS
sg. 172800 NS AUTH02.NS.UU.NET.
sg. 172800 NS NS.RIPE.NET.
sg. 172800 NS
sg. 172800 NS

AUTH02.NS.UU.NET. 172800 A
NS.RIPE.NET. 172800 A 172800 A 172800 A

;; Total query time: 253 msec
;; WHEN: Mon Sep 21 18:58:24 1998
;; MSG SIZE sent: 20 rcvd: 237