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.
What is a back door? Code in software, hardware or firmware that gives remote access to unauthorized parties.
How do remote parties connect to your backdoor’s when these servers that don’t have access to the open internet?
The most plausible scenario is the one @Phead128 described - some type of logic bomb where specially crafted training data or driver update instructs the GPU to perform bad calculations or premature failures.
So yes, there could be something bad and hidden in the GPU servers, but calling them a backdoor isn’t the right technical term
