I'd guess that it's actually a requirement. Domains registered under ccTLDs (country-code top level domains) are managed by regional bodies[0]. In case of .su, it's either RIPN or ROSNIIROS[1] which manages the registry and registrations.
But again - IANA is universally queried about given TLD and redirects the query to regional body. In case of .su:
> whois yandex.su
% IANA WHOIS server
% for more information on IANA, visit http://www.iana.org
% This query returned 1 object
refer: whois.tcinet.ru
domain: SU
organisation: Russian Institute for Development of Public Networks
organisation: (ROSNIIROS)
(...)
# whois.tcinet.ru
% TCI Whois Service. Terms of use:
% https://tcinet.ru/documents/whois_ru_rf.pdf (in Russian)
% https://tcinet.ru/documents/whois_su.pdf (in Russian)
domain: YANDEX.SU
nserver: ns1.snparking.ru.
(...)
state: REGISTERED, DELEGATED
[0] USUALLY; .tk, .ga, .cf, .gq were managed by by infamous Freenom due to incapability of owners to do it themselves
I think it's actually fairly common to delegate, no? .io was ran by just one guy at some point (or so I've heard). .us is run by GoDaddy (but I think the US government does keep them in check).
But again - IANA is universally queried about given TLD and redirects the query to regional body. In case of .su:
[0] USUALLY; .tk, .ga, .cf, .gq were managed by by infamous Freenom due to incapability of owners to do it themselves[1] https://www.iana.org/domains/root/db/su.html