That's a bullshit video. The guy conducting the tests doesn't know enough about how SSDs work or about how Macs boot to be drawing conclusions, especially not such strongly negative conclusions. He would have had to at the very least attempt a tethered restore from DFU (as is done for these phone storage swaps or any other situation where the NAND is blank or thoroughly corrupted) before concluding that Apple has everything totally locked down.
Adding the second drive module is something that we should not expect to work the way he tested it. Plugging in a second hard drive to a desktop does not magically give you a RAID-0 without extra steps; expecting this from SSDs doesn't make any more sense. And without reverse engineering the machine config from the quoted prices, we don't even know if the two modules had the same capacity, let alone the same type and manufacturer of NAND (which is probably important if the system is going to treat the two modules as one SSD instead of exposing them to the OS as one drive).
> Adding the second drive module is something that we should not expect to work
Hard disagree. This approach works for just about any normal storage medium. Apple decided to Think Different about where the SSD controller lives and works and how storage pinouts work, but their weird approach wasn't publicly documented anywhere until people like this guy started trying.
Uninitialised storage would've normally showed up just like that, as uninitialised storage, ready to be partitioned. Instead, the device didn't even turn on, let alone show the added flash storage in a partitioning tool.
You're still thinking of the storage modules as standalone SSDs to be managed by software, rather than devices hidden behind hardware RAID.
I did almost exactly the same kind of drive-swapping tests with the WD Black AN1500 [0] in an Intel PC, with the same results in terms of whether the system can boot from the device, but none of that supports the conclusion that Western Digital is locking down what drives can be used behind that RAID controller. You can swap drives on the AN1500, but almost any such operation will require you to erase and reinstall.
Confusion is understandable here because such technicalities were not officially documented and the community had not figured it out at such an early point.
https://www.youtube.com/watch?v=xEwS_VGD2yY&t=620s