Try to explain how the hidden content would be found? It effectively doesn’t exist until the password is in place. It isn’t hidden, it isn’t connected at all.
SD card the author says was added to offset the cost of eMMC but would presumably stand out as anomalous with whatever AI is classifying the scan
But saying that, the sheer amount of fake flash storage out there now, that's just a glorified USB-SD reader in a box.