I'd be interested in further details about the testing. If any manufacturer actively wants to backdoor their hardware I'm skeptical that anything but an extremely expensive teardown of an infected device would find it.
It is simply incorrect to imply that reading vendor provided source can usefully decrease the possibility of a targeted attack. Comparing (hardware provided?) software checksums is not a real improvement. Juxtaposed with your "interest" in the topic, such an argument naturally arouses suspicion (sorry).
There is obviously nothing "wrong" with being interested in this fascinating clash of powerful interests, the amount of interest each discussion gets shows you are not alone.
So I'm not just hammering at what you've said, I'll make my own statement: There's absolutely nothing you can do to defend against a motivated attacker providing you with complex computer hardware (let's say anything that has software/firmware). Corollary: It's a fool's game to use hardware from those whose interests conflict with your own.
China and the US have a massive conflict of their interests. Each should not use hardware provided by the other. The risk for each is real and unavoidable.