Yeah, you nailed it. The latest reporting on this says that Apple isn’t paying them yet, because they think OpenAI will get more benefit out of just having their product in everyone’s faces:

Apple isn’t paying OpenAI as part of the partnership, said the people, who asked not to be identified because the deal terms are private. Instead, Apple believes pushing OpenAI’s brand and technology to hundreds of millions of its devices is of equal or greater value than monetary payments, these people said. Source.