Nobody knows anything for certain about literally any consumer hardware in existence, because silicon is not end user introspectable. If you want to go there yo

Nobody knows anything for certain about literally any consumer hardware in existence, because silicon is not end user introspectable. If you want to go there you have to look at Precursor (which uses an FPGA and can make the claim that a backdoor is infeasible in that architecture). But what we can say is that we know that Intel has hyper-privileged backdoor modes and coprocessors, while every single thing I've seen so far about the M1 indicates it was carefully designed to isolate all coprocessors from the main CPU. Everything is either behind an IOMMU or can't do DMA at all or has some other form of address filter. And knowing what I know about Apple's security posture, it's entirely logical that they designed it this way.