A non-open system is not verifyable and therefore not trustable. Therefore a non-open system can never deliver privacy. At best it can attempt to trick you into believing it does.
Risk isn’t an absolute. Open systems may be verifiable, but they are also more difficult to use, inconvenient, and lack the features users want. So most people won’t use them or will use them badly. Apple reduces privacy risk relative to open solutions used by non-expert users. The purist approach to privacy increases risk to ordinary users. It’s better to be pragmatic; Apple isn’t ideal, but it’s better than the realistic alternatives.