You seem to forget how difficult it is to manufacture a single unit of a mother board to achieve the effect you're imagining. Do you really think they have a production line where every week or so they create a different mother board adding this circuit? Why in the world would they create these additional costs, and is it even economically justifiable? This whole story seems each day worse, just like believing in Big Foot.
The whole point of inserting malicious hardware for shadowy intelligence-related reasons is that you do it either out of government coercion or via government funding.
I don't know why it's so implausible that Chinese intelligence is using the Chinese tech industry to carry out their operations when American intelligence uses the American tech industry to carry out their operations. Nobody said PRISM was fake news because why would Apple and Google and Facebook invest in it; the actual investment came from NSA and the cooperation of these companies is via top secret FISA warrants. China doesn't even need to maintain the pretense of legality that the NSA does.
Again, I don't think the Bloomberg story by itself is credible anymore, but I also don't think it's as implausible as you are making it sound.
If the board house has copies of the gerbers, which they should, someone could easily edit those to introduce a spot for the implant. And if they already have all of the drawings, they can just add those empty pads into the design so any inspection wouldn't raise alarm. Then install the actual implant post-inspection. Maybe they get lucky by just re-purposing unused pads from the original design and route the traces to where ever.
