We understood all that already.
However, we said, you can use Time.setTime() in your code to feed any time into the onboard RTC and the devices will keep updating the time henceforth.
So in order to mimik the cloud time sync, you just need to tell all devices at once what the current time is, they all will update their local RTCs to that time almost synchronously (providing your code is written properly with minimal latency) and you'll have a fleet of time synced devices.
If you are already communicating TCP or UDP with your devices, you should know how to address all devices simultaneously 