The second generation chips mentioned in the Bloomberg article were fitted inside the plastic of the motherboard. I don't think it's a stretch to imagine that such chips might be really difficult to spot. And if they aren't the natural next step for a nation state is to build an xx billion dollar fab factory and produce smd "resistors" that contain one of these chips.

I think the real trick is a real covert channel that's hard to find both analytically and via stastics/machine learning.