Clause 12 exists because the rules were written by lawyers, in their mind it never hurts to codify the same thing multiple times.
The immunity of international organizations, similar to sovereign immunity, is based on customary international law, and usually codified into national law, eg the US INTERNATIONAL ORGANIZATIONS IMMUNITIES ACT, 29 DECEMBER 1945.
If you can't find the information, try a google search

This paper is a good place to start if you want a current update on nitty gritty aspects of
IMMUNITY OF INTERNATIONAL ORGANISATIONS AND ALTERNATIVE REMEDIES.
Sorry for the shouting here, the upper case is just copy and paste.
As for WIPO, their relationship with Switzerland (including Swiss courts), is governed by the
Agreement on Privileges and Immunities of the United Nations concluded on 19 April 1946
You would not be the first to try to take the UN to court, and I am no expert on this matter, but I can assure you that your chances are as close to non-existent as it gets.
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