Thinking about it some more, there are actually some solid benefits to using a hardware implant rather than compromising the firmware or, say, a flash IC.
It's conceivable that there's some exploit that requires a fairly minimal modification of the firmware binary, and that the location of that modification is easily recognized within the binary by its surroundings. As long as that specific area of the binary was not changed (which could be unlikely unless that specific area of the codebase was changed), then the malicious device could be capable of compromising any new firmware version, even if the targeted area appears somewhere else within the binary. Not unlike the infamous
Ken Thompson hack. Even pulling the flash from the board and dumping it externally wouldn't reveal anything amiss. You'd have to directly sniff the traffic between the embedded controller and the interloper to capture the change to the binary, and even then it's conceivable that the interloper has some sort of context awareness to help avoid detection (not unlike the VW firmware that could detect emissions testing).
Also, somewhat ironically, the fact that almost everyone here is saying that it makes so much more sense to compromise the firmware or one of the existing ICs on the board is something of an argument for NOT doing it either of those ways--after all, it's exactly what anyone would expect! It would be far sneakier to make a fake passive component that pwns the board because that's such a ridiculous idea that no one would ever bother to do that sort of thing, right? Just like no one would try to cram a network traffic siphon with a built-in RF transceiver inside of a network jack. . . .
I also think a lot of people are overestimating how easily an extra component or two would be detected. I mean, I sure as hell wouldn't notice an extra couple of passives on one of my boards between finished a design and receiving the assembled thing, and my boards aren't nearly as complex as a server motherboard. Plus you would have teams of people working on those things, and no one person is going to know the entire board like the back of their hand. They're only going to start comparing the finished board to the assembly drawings if something doesn't work, and even then the discrepancy won't be caught if that work is happening at the contractor that installed the malicious parts in the first place. It all depends on how much of the work Super Micro is farming out, but I imagine that they have their design process down to such a science that it's very rare they have to do component-level debugging.
Of course without more information it's impossible to tell if these benefits likely outweighed the difficulty of implementing the exploit in the way that Bloomberg describes, but still, it's all plausible through a certain lens, which is what makes it so intriguing.