The general case of hardware being backdoored is believable, but the problems come down to, not just the lack of any kind of corroborating evidence, but the nature of these specific claims themselves having some hard-to-believe holes in them.
Basic informational hygiene is that the claim is garbage until proven otherwise, "credibility" notwithstanding. There is not only not a single positive reason to believe this story, there is mounting evidence that it should not be believed.
Haha, "credibility". A thing which does not exist.
Seriously, though, it seems that your idea of "basic informational hygiene" conflicts with a basic security posture in this case. We don't have to assume Super Micro has never been hacked, so I don't know why we would assume that. More in keeping with the topic of this thread, we don't have to assume the State Department (or whoever) has never caused a story to be published or discredited, so I don't know why we would assume that.
We're on the internet, a medium in which information can be trivially exchanged. Easily-defeated heuristics like "authority" and "credibility" are meaningless, if not harmful, when individual claims can (and should!) be evaluated on their own merits.
Basic security posture, sure, but nobody's arguing that we should change that and pretend that Supermicro is completely safe. Nothing is ever completely safe.
..but we're talking about a very specific claim which already has a number of gaping holes blown into it.
https://www.servethehome.com/investigating-implausible-bloom...
Basic informational hygiene is that the claim is garbage until proven otherwise, "credibility" notwithstanding. There is not only not a single positive reason to believe this story, there is mounting evidence that it should not be believed.