I think the author is over-simplifying here a deal. Page 99 here. https://manuals.info.apple.com/MANUALS/1000/MA1902/en_US/apple-platform-security-guide.pdf ht

I think the author is over-simplifying here a deal. Page 99 here. https://manuals.info.apple.com/MANUALS/1000/MA1902/en_US/apple-platform-security-guide.pdf https://manuals.info.apple.com/MANUALS/1000/MA1902/en_US/app... Which describes that the private keys are generated and held on device. Now I think this model *may change when you enable icloud messaging, in which case an encrypted messaging key may be stored in your icloud account. So you may opt to have apple store it, but in an encrypted manner that they can't undo. This part is a bit speculative on my part though, so grain of salt.