Bloomberg's claim is that a miniature device used for RF analog electronics was coopted and inserted into a board that would never have such a part designed in. This requires modifying the board artwork, the pick and place config, any automated inspection and test equipment, and adding a foreign part reel to the supply chain.
It is much easier to compromise firmware directly or modify ICs that are already part of the design. The risk of being caught is much lower and it would be stupid to attempt anything more elaborate.
Or, they could run their own fabrication facilty, where they can exert total control over the production line, in total secrecy, and you'd never know the difference, or notice a sacrifice in the fidelity of replication.
Think that's impossible? Not at the nation-state level. Not in communist countries where everything is property of the government by default. Not in capitalist countries like the United States, where entire nuclear facilities are replicated in secret. [0]
Not in capitalist countries like the United States where you can just contract a manufacturer to produce the board you want. Even parts acquisition becomes a job to be done. As long as the producer dosn't know (or even does know but can be silenced at the right level) then really, once the actual design is in hand and assuming the parts aren't too hard to get there's little to stop someone from producing a board just like a board produced elsewhere.
It is much easier to compromise firmware directly or modify ICs that are already part of the design. The risk of being caught is much lower and it would be stupid to attempt anything more elaborate.