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

[1] https://www.iana.org/domains/root/db/su.html