Newsom has to lose the "Should we recall Newsom" vote first, and that's gonna be a tall order. In 2003, Gray Davis had a ~30% approval rating and most people wanted him out. Newsom's approval is in the 50s IIRC and most polls have him winning the recall vote with a mid-50s margin. Davis also had his lieutenant governor run in the recall election, signaling he was unpopular in his own party. Newsom, on the other hand, has locked down the party establishment and even his critics on the left. Bernie Sanders, for example, wrote an op-ed defending Newsom and calling the recall effort a right-wing hit job.

Never say never obviously - if Newsom loses the "Should we recall Newsom" question then Jenner could definitely win. But Jenner isn't running against Newsom head-to-head. Newsom's running against himself, and Jenner's just waiting in the wings if he fails.