Apple has shown time and time again that they have the human capital and money to tackle massive projects discretely. It's already fairly well known that Apple's NLP experts from Siri have been reallocated to some secret project. They are more than capable of training an LLM but given their track record in other segments they probably want to wait for the technology to become more "polished" and give less hallucinated answers. They are likely also want the LLM to work locally (at least partially) on their devices using the Neural Engine which adds further engineering complexity to their project. They could even be timing the LLM's launch around a hardware release capable of running the model (M3, M4, etc...).