I don't know anything about this issue but ignorance has never stopped me from weighing in before. Why would the Chinese take the easiest way to plant spying capability if they wanted it to go undetected? They certainly use those other techniques but why assume they would never do something more complex but better disguised?
Because the easier ways ARE better disguised. . . and a hell of a lot cheaper to do than redesign an entire motherboard from scratch that will be as obvious as all hell because it sits out there for everyone to find, sticking out like an unwelcome wart on a nose. As the security expert in the article pointed out, it would be trivial to hide an extra circuit on a flash memory drive or even hidden in a memory chip which did the malicious operations while still functioning as designed. The only way to tell it had the malicious section would be to put it under a scanning electron microscope and find the extra circuitry. It would not be obviously sticking out on a circuit board. This was a stupid approach.
Another is to hid it in firmware. . . software that tells the existing processor how to do its work. . . and just use the ordinary hardware already existing on the computer to do something extra during idle time. Cheap, effective. Extremely hard to find.
On “spy stuff” it’s best to just not believe amything public. Not the claims, not the denials; there are too many good reasons for everyone to lie.