You're just saying your average app programmer or 3rd year writing Pyhton can't build backdoors, and that's true, but real backdoors operate at microcode level, data that gets injected by the separate hardware on the chip, between the compute unit and I/O unit, or just code that make use of vulnerabilities that might or might not be known. Saying firewalls can prevent backdoors is like saying you're safe from spying because you installed a lock on your door: someone who want to spy on you ain't knocking asking to be let in.
That's not to say having controlled hardware everywhere is meaningless, it certainly makes the job much harder if every layer between the GPU and reporting server runs on Huawei metal, but again counter-espionage isn't about what you know, its protecting against what you don't, and the only way to protect against what you don't know is not use Nvidia chips to begin with.
As for reputation, lol, you're talking about people who openly brag about installing backdoors, these are not people who care about reputation at home, and they're also people who already lost their reputation from China's perspective, e.g. Nvidia might get banned if they have backdoors, aannd they were banned.
