> Modern ASICs are so complex that I'm sure that sneaking a tiny backdoor into the behemoth that's a modern CPU or embedded SoC would be almost trivial.

I suspect putting in a backdoor would be difficult because they are complex. Wouldn’t it be far too easy for the backdoor to inadvertently cause reliability or performance issues? And the bug would have to be useful enough to warrant potentially destroying the semiconductor business of a nation, not just some difficult to trigger side channel.