Let's drift into some wild speculation here.
Suppose this was a malicious attack, and PRC intelligence within SuperMicro and perhaps even Apple are responsible for these machines getting shipped and installed by Apple. Let's further suppose that the plot was uncovered by US counterintelligence.
Why the heck would US counterintelligence operate inside of Apple, discover security vulnerabilities that affect Apple in particular, hide this discovery from Apple's senior management, and then go on to leak it to Bloomberg while continuing to cover it up on the inside of Apple?
There is a reasonable-ish explanation for this. US counterintelligence usually falls under the purview of the FBI, and the FBI has had a couple public spats with Tim Cook over the years. Given Apple's commitment to user privacy, it's reasonable to presume that Tim Cook would not knowingly allow the FBI to infiltrate Apple. It's also reasonable to assume that, given any number of motives both virtuous and vicious, the FBI would have a vested interest in infiltrating Apple. In this scenario, notifying Apple management about these issues would risk revealing sources and methods.
At this point, the only remaining question is: why wouldn't the moles just quietly inform Apple management about the issue? Clearly, Apple personnel who are in the business of receiving and installing server hardware would have a defensible cover for poking around and raising questions about weird chips that mysteriously appeared on the hardware. There are possible explanations for this, but at that point we're just compounding conspiracy theories on conspiracy theories.