Ben Thompson on Apple joining and then leaving the OpenAI board:

…joining the board of OpenAI emphasizes the narrative that Apple needs OpenAI, suggests that OpenAI is far more integral to Apple Intelligence than it actually is, and puts Apple in the firing line for any future OpenAI controversies. This was a bad idea, and the company is right to back out.

In the short term, it sounds like the OpenAI integration in iOS 18 will ship months before the full Siri + Apple Intelligence is ready. So you could say Apple does need them. Long term, there will be less and less need for a partner.