However, to process more sophisticated requests, Apple Intelligence needs to be able to enlist help from larger, more complex models in the cloud. For these cloud requests to live up to the security and privacy guarantees that our users expect from our devices, the traditional cloud service security model isn't a viable starting point. Instead, we need to bring our industry-leading device security model, for the first time ever, to the cloud.

As stated above, Private cloud compute has nothing to do with the OS level AI itself. ರ⁠_⁠ರ That's in the cloud not on device.

While we’re publishing the binary images of every production PCC build, to further aid research we will periodically also publish a subset of the security-critical PCC source code.

As stated here, it still has the same issue of not being 100% verifiable, they only publish a few code snippets they deam "security-critical", it doesn't allow us to verify the handling of user data.

• It’s difficult to provide runtime transparency for AI in the cloud.
  Cloud AI services are opaque: providers do not typically specify details of the software stack they are using to run their services, and those details are often considered proprietary. Even if a cloud AI service relied only on open source software, which is inspectable by security researchers, there is no widely deployed way for a user device (or browser) to confirm that the service it’s connecting to is running an unmodified version of the software that it purports to run, or to detect that the software running on the service has changed.

Adding to what it says here, if the on device AI is compromised in anyway, be it from an attacker or Apple themselves then PCC is rendered irrelevant regardless if PCC were open source or not.

Additionally, I'll raise the issue that this entire blog is nothing but just that a blog, nothing stated here is legally binding, so any claims of how they handled user data is irrelevant and can easily be dismissed as marketing.