The "should have" pressed the sources for hard proof, or found examples in the wild of adulterated servers before publishing the story. That ship has sailed, of course.
But if what was reported was actually true the best thing that can happen now, for Bloomberg and the public, is for their sources to step forward voluntarily with proof.
Admittedly, that will take a lot of courage in today's political climate, but if it's not all just bullshit, highly principled people may do it.
What I would like to know is what the Bloomberg reporters expected would happen as a result of this story?? Did they think Supermicro and Apple would just admit it? Then what?
If the sources are willing to step forward now, why wouldn't they before?
Sometimes the reason for reporting evidence you have is to motivate other people to display evidence they have but don't know was interesting or useful until they saw your story.
That's why so often a single public allegation leads to an avalanche of similar allegations.
That would be a mistake. Don't just throw your sources to the wolves if you couldn't prove their story.
They should press their sources for proof, or stronger evidence. If they can't find any then they should issue a retraction.