That might be their internal reasoning but Apple will very quickly move to have these capabilities in house. Apple has been working on machine learning for a while but they don’t collect data so they are unable to build these LLMs.
For now it makes sense for Apple to leave the liability of basing these LLMs on copyrighted data. If OpenAI losses those court battles, they take the hit for services rendered to Apple. None of that liability transfers to Apple.
Meanwhile, Apple is going about this the Apple way by encouraging developers to integrate their apps into new frameworks being added. This gives them access to user data directly from the source allowing them to build personalized models.
These models will likely be far more useful to the day to day mundanity of life than the hallucinogenic encyclopedia that is ChatGPT.