A British man is ridiculously attempting to sue Apple following a divorce, caused by his wife finding messages to a prostitute he deleted from his iPhone that were still accessible on an iMac.
In the last years of his marriage, a man referred to as "Richard" started to use the services of prostitutes, without his wife's knowledge. To try and keep the communications secret, he used iMessages on his iPhone, but then deleted the messages.
Despite being careful on his iPhone to cover his tracks, he didn't count on Apple's ecosystem automatically synchronizing his messaging history with the family iMac. Apparently, he wasn't careful enough to use Family Sharing for iCloud, or discrete user accounts on the Mac.
The Times reports the wife saw the message when she opened iMessage on the iMac. She also saw years of messages to prostitutes, revealing a long period of infidelity by her husband.
Yep, instant sync is never a guarantee. There still has to be a queue for command messages along with authentication plus authorization of said commands. And just like you said, you must be connected to a network that then can reach their cloud to even receive the command queue.
I run a sync service between multiple Active Directory domains as a result of a merger and the directories haven't been cutover yet. Along with this sync is a password sync that is normally instant. Most of the times (> 90%), less than a second. Sometimes 3 seconds. Other times? 2 minutes. Even when things are within the same LAN, there's the possibility of a backed up queue.
So yeah, this is purely on him trusting the sync implicitly and not verifying. In my case, I trust it too but will on occasion have to assist users because it's not infallible. Karma got him and I have zero sympathy.