That's true. While yes the WHO is following regulations, they're following regulations that were created to exclude the Republic of China from it in order to entice the PRC to join it. The PRC gave multinational organizations such as the WHO this condition for accepting the invitation to join, and multinationals greatly desired the membership of the PRC.

The WHO had the choice: accept the Chinese Communist Party's demand to exclude Taiwan, or be shunned by the PRC. The problem isn't necessarily with the WHO, it's with the CCP.