One could not trust anything from Bloomberg's for years now. Sad really, but to be expected from misguided capitalistic incentives for the lyricists at Bloomberg. Did the Chinese get caught in a single case? Maybe. But make no mistake, every agency with resources does this, if they think the information to be gained is worth it, and they are pretty sure they know where such a modified server will end up at. You just don't read it in the press, journalists and spooks have had a long tradition of a mutually beneficial alliance in the West. You read about it when it suits everyone. Everyone but the other side of course. Also this is not a measure sprayed willy nilly and then look where the servers end up. Only to find your implant now lives at a Texan gardening company, that needed an upgrade.
I haven't researched it but I can imagine there is political ambition to withdraw a chunk of manufacturing from China, to lessen our tech dependency on this country and its leadership. I hear Vietnam is nice. India also on the short list.
Also I don't think Supermicro was targetted accidentally. If you look at HPE's iLO 5 for example, their firmwares are signed and verified using public keys embedded in chip's ROM. Much harder to subvert than some simple AST2x00 off-the-shelf chips which Supermicro likes to use.
The good news is systems and operating systems appear to be so secure now, that such implants are necessary. Or they just don't have a suitably high up asset at their target. I doubt China or any of the other Top 20 do not have enough money to just buy someone working at Apple. But maybe they can't, because Apple already pays the important engineers so damn well, that this incentive is gone, vis a vis the consequences if uncovered.