The PIN number is entangled with the device's 256-bit "UID", which itself is on-die in the SoC/CPU and NOT extractable without either being able to run code on the CPU, or decapping the SoC, reverse engineering its implementation, and extracting the UID, all from the SEM imagery.
The PIN number and the UID are fed to key derivation code for strengthening; the result of that process is used to actually perform encryption of the data on the NAND.
The weak point here is the PIN number; the FBI would be extremely hard-pressed to brute force the derived AES keys.
This is described in the "Tangling" definition on page 59 of the iOS Security Guide: https://www.apple.com/business/docs/iOS_Security_Guide.pdf
I flagged the article, as the entire argument is predicated on a factually false premise.