Xray inspection by a human specialist

Further in the article, Bloomberg mentions a chip, smaller than a pencil tip, sandwiched inside the PCB itself, under other traces. That seems bananas difficult to pull off, but could x-ray inspection find such a thing if it existed?

Yes, because the incident they reference was a mid-PCB-layer chip discovered by govt agencies using X-rays. The findings were presented at a private conference in Virginia. Sources who attended said "they've seen the xray pictures" etc.

I specifically mention non-automatic inspection as automatic one may well not have the "paranoid mode," unlike a human who specifically told to go over every individual square millimetre.

Minimum quantity orders for these servers are typically 10,000.

Even at 1,000, I don't see how humans could reliably detect this.