Re: I give SuperMicro the benefit of the doubt.
If the entity making a claim can not or will not produce proof of their claim, then the other party can legally sue for libel. If the paper refuses to produce one of the supposedly compromised boards, SuperMicro can take them to court & sue the shit out of them.
They can try. However if it goes to court the onus is on the party making the claim to provide the evidence - in libel cases this is inverted to how you might expect, i.e. the potentially libelled party has to prove they were libelled rather than the libelling party proving what they have said is true, since the case is about the libel rather than the original allegation.
That makes the case difficult to prove absent evidence of conspiracy or some other skullduggery since you are trying to prove a negative - the inability to produce boards that do not exist is not itself evidence that the supposed boards do not exist.