To the degree that Chinese manufacturers robotically use our layouts and mask sets there is little danger of them using our designed in back doors.
Unfortunately we train a LOT of Chinese engineering students in our universities, and SOME of them are PLA officers, and SOME of them are quite capable of reverse engineering our designs to insert any back doors the Chinese government desires.
The rules of threat assessment call for identifying what an enemy is capable of doing, not necessarily what they are actually doing.
They are capable of inserting a dormant shut down code in every microprocessor they manufacture, some of those chips might work their ways into critical systems. Most will end up sprinkled throughout our electronic infrastructure.
How much havoc would ensue if say, 10% of our gas pumps, heart monitors TV stations and home shopping computers all died in the same hour is an exercise left to the reader...
Of course that's correct....but....it's sooo much easier and more effective to "insert a test mode" via a commie sympathizer sellout round-eye in San Jose or Austin!
I'm sure you recognize that it's exceedingly hard and dangerous to slap in some hard logic into a mask and have it not only work, but not screw up other functionality or test vectors, risking discovery.
It's far easier to feed it in at the top of the food chain in HDL as a "test register" right out "in the open." Except "in the open" might be well documented on Page 1,442 of the design datasheet, which is an internal Intel or Freescale document that nobody in Federal Procurement will ever see, much less read.