How does this defend against a the type of hardware-based attack discussed in the article?
A proper chain of trust that starts on the BMC chip could absolutely protect against that. At that point any modification to the boot image would leave the BMC refusing to boot rather giving attacker control.
It would significantly raise the cost and difficulty of this sort of attack.
In my opinion, modifying the board layout with the additional chip and modifying the production process for the server boards stealthily already has a pretty high cost and difficulty.
That's true, but properly implemented secure boot could serve to increase it by an order of magnitude.