Never a plausible story in the first place
This was never a plausible story from day one. SuperMicro's manufacturing facilities will work from downstream manufacturing artifacts e.g the Gerbers, pick and place files etc, which drive the PCB manufacture and subsequent pick and place and wave soldering stages and so on. They don't have the original design files, nor do they need them to manufacture product.
It's like 3d printing. I have the FreeCAD files, I export STLs, the STLs get sliced to produce gcode. I give the gcode to the printer, that's all it needs.
Yes - theoretically I can hack the gcode; that's hard. And detectable, if I compare with a master copy. Much easier to change the CAD file - but that's not a downstream artifact, why would I give that to the people printing my design?
To somehow interfere with these manufacturing files, which are under rigorous version control, and distribute the tampered copies to the fabrication pipeline would be an incredibly challenging task. Since QA also then take finished boards and X-Ray them and compare against an exemplar board, you also then have to somehow ensure that the resulting compromised board is undetectably different, which given that you need to route power and signal traces to these spy chips really starts to challenge credulity.
Bloomberg could make these assertions because their reporters and editors were technically naive about hardware manufacturing and probably figured if you can tamper with software, surely it's just as easy to tamper with hardware. As for why Bloomberg made the claims, a number of industry sources believe that they were indeed the fall guy for a state-sponsored disinformation campaign timed to coincide with the US disfavouring Chinese vendors. Recall that AT&T got leaned on heavily over selling Huawei phones to the extent they pulled out from a deal.
Finally, no-one's sueing anyone because, in the case of Apple/Amazon there's no basis for legal action; no-one accused them of anything but, allegedly, being victims. They deny this. End of story. As for SuperMicro, they could sue Bloomberg but, absent proof of malice, Bloomberg have a fairly strong defence that they had reasonable grounds to believe the story to be true.