Seems more problematic though. You'd have to manufacture the doctored boards, extract them from the normal shipping process, keep them hidden somewhere, then swap them out for the ones destined for the target customer(s). I guess it could be done, but it seems risky.
Couldn't it be done on-demand? Apple orders X hundred boards, motherboard manufacturer makes their small modification(s) to a line that is currently producing the same models of motherboard as Apple ordered, they produce a handful, then they revert and mix in a few of those modified boards into the real order. I don't really know the exact scale, so maybe they make a few hundred / the entire order with chips in them, but economic cost isn't a big deal for things like this, so even losing money making the modified boards wouldn't be the end of the world (and presumably they get a hefty sum of money for whoever is paying them to do this).
I thought China was famous for extremely short turnarounds for industrial engineering edits, so it seems plausible that they could manufacture the boards in a reactionary way and not need to do much in the way of logistics to get them to their targets.
