The reason that it's unlikely that there are tons of phones attached to something with lightning cables flashing is simple: the devices need to get put into the correct box, because the box has the IMEI, SN, MAC addresses, etc. Having a huge flashing station would make that process difficult, since none of that information is actually on the phone itself anymore. So if one phone is out of order the whole mess is out-of-order, and resolving that would be extremely difficult.
I'm starting to think that the only way for them to do this is for the bootstrap process to download the image, then report back when done. That's the only way you could pull unfleshed phones off the line in a timely manner. It could also be that the process is also in charge of telling the device what its IMEI, SN, MAC, etc are, so they can get burned in at that time.