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.
A chip smaller than a grain of rice nested between motherboard layers.
What tools could detect such a chip?
While I don't doubt Apple inspects, the zero-day threat always exists.