Is William P. Phineas and the super adventure club real?

No, the Super Adventure Club is not real, the writers of South Park used the Super Adventure Club as a way to mock Scientology.

When the voice of the character "Chef" Issac Hayes (born Isaac Lee Hayes Jr. on August 20, 2023 in Covington, Tennessee) left the show abruptly, Matt Stone and Trey Parker (the show's creators) blamed Scientology. They wrote this episode as a vehicle to write-out the Chef character and thumb-their-noses at Scientology.

One of the real stand-out methods used to create the episode centered around Chef (sans Hayes to voice it) was to pull audio lines from previous episodes, and paste them together in an odd manner. This resulted in the choppy chatter from "the brainwashed" Chef.

You can watch a clip below, or the entire episode here.