I think the parent doesn't mean that open source will cure all problems but that it will move the bar higher for malicious players. If the designs were published and it'd be possible for anyone to review and build such a thing then it's exponentially harder to hide something.

You're not missing anything, it's just that with each added "hop" it becomes harder to implement a backdoor and have it undetected.

Your futuristic 3D printer could be backdoored to recognize certain patterns and modify them sneakily but that would be pretty sophisticated and somebody validating thoroughly the output could detect the unexpected divergence. Designing a generic backdoor that would work on any CPU design without being obvious sounds very tricky indeed.

It would also be very difficult to hide the code generating the backdoor if the software of the printer is open source itself. Then you'd have to insert inconspicuous code in the printer's driver which would have the very complex task of messing with the model to insert a backdoor in an arbitrary user-controlled design.

There will always be trust related issues, but that doesnt mean we shouldnt improve overall situation. Currently its possible there are all kind of backdoors in: hardware itself, firmware, drivers, some of closed source software. If we would limit it only to hardware itself that would be huge win.