Every time you take your car to be serviced by tge dealer it’s plugged into a diagnostics computer which reads the ECU, with the price of storage it is entirely possible that disabling the cell connection just causes the ECU to write it to local storage for upload at service read. The diagnostics machines are definitely connected to manufacturer servers.
Doing so is trivially easy the telematics is going to be caching before sending, all you need to do is manufacture that cache storage to be large enough (and it’s flatfiles we’re talking megs not gigs) and tell the software not to delete until it has an an acknowledged receipt of transfer.