I am more inclined to believe Super Micro at this point. It is not just them that are denying this. Apple and Amazon have denied it as well and Apple even went so far as to write a letter to Congress[0]. With a hardware attack like the one that Bloomberg reported, there should be some actual physical evidence out in the wild but nothing has been found to date. I think that they really failed with regards to due diligence.

[0]: https://www.scribd.com/document/390401381/Letter-October-8th...