The firmware is stored on the primary storage of the computer. Which is absent if you are replacing it. So when you put new primary storage in the machine, a

The firmware is stored on the primary storage of the computer. Which is absent if you are replacing it. So when you put new primary storage in the machine, a new firmware that is cryptographically signed with the hardware keys that are unique to the T2 in each computer are used. If they didn't use Apple Configurator running on a second Mac for this they would have to put all of that functionality in firmware on the machine somewhere. Driving up cost and complexity for an obscure task on a low volume machine in the first place. Yes, sounds like a valid use of engineering resources to overcome the "hardship" of having access to a second Mac for the 10 minutes (or less) the swap requires.