It sounds like the work on the privacy layer was significant and to give "equal" access to other competing AI systems, they would need to include that "for free" as part of the platform. Or they could try to keep that as the moat for Siri AI, and only offer privacy "entry points" that other agents can tie into, but vendors would have to implement privacy preserving functions themselves.

This is the bit that's likely hard, because generally keeping safety and privacy guarantees as data flows through the system is extremely hard, and Apple would not be able to guarantee it for other products without large review investment.

But ultimately, they probably just do not want to do it until Siri AI gets a decent marketshare first, so competing agents would have to both build new solutions for the platform once open, but also deal with an incumbent dominant player already on people's phones.