That's complete nonsense.
Dynamic dispatch has nothing to do with the ability or not of a program to access API. Dynamic dispatch is the selection at runtime of the correct version of a polymorphic function. Obviously, you can sandbox programs written in languages using dynamic dispatch.
I am sure Apple would love to know how you've managed to solve this.
The point of a private API not security, it's to distinguish between the public interface that is meant to be stable and implementation details that might change.
They might do some rudimentary checks to catch obvious usage of private APIs, but it's not part of the security model and still apps show up on the App Store that use private APIs, all the time.
but there is nothing intrinsic to their operation that requires it, and apple could un-plug it.
this is like apple arguing that IE is central to the fabric of windows, and can't be removed during the european antitrust suit.
it's dishonest, but apple will likely make the same claim.
And many of those APIs can be used to extract enough information to fingerprint the device, determine your location or steal your data e.g. accessing the list of WiFi networks or browser history.
So no. The two concerns are very much related.