IC=Instrument Cluster, MCU=Media Control Unit, CID=T3 daughtercard
Yep, that would do it.
When the MCU initializes it starts a boot coprocessor in the Tegra3, which looks to the first address in the Spansion flash (on obverse of the CID) to boot. Early in its boot process (equivalent to its BIOS) the coprocessor turns on code-signing, which thereafter checks that every ECU (Electronic Control Unit) in the car is running the same version of firmware.
If you put your own car's daughtercard into the bench MCU, or the bench MCU's daughtercard into your car, black screen due to firmware version mismatch. (as confirmed by code-signing) Don't ask me how I know...
I have permission from Sid to publish his custom Fusee Gelee hack for Tesla to turn off code-signing, but have no code nor further info.
And rats, I muffed that amusing metaphor; it should have been 'Carpet match the drapes?'. (referring to a woman's hair color)