To that point - I generally think that engineers at ostensibly-privacy-minded companies like Apple are competent, well-intentioned, and good canaries. If I were to open Twitter and see a lot of people "seeking new opportunities" from Apple's security team and not able to give their reasons? It's very possible that a backdoor was built contrary to public statements, and they could not condone the discrepancy.
But here, not only is the list of hashes editable with merely a configuration change, but it is fundamentally a list of hashes that is designed to be secret and non-auditable and supplied by a non-auditable supply chain. In fact, the proponents of this program would argue "don't give the Apple engineers and product managers access to the hash list, nor access to whether test images are matched by the hash list, because it could be used for nefarious purposes if they themselves are perpetrators."
So at any time a photograph commonly used to criticize a regime or commemorate a specific event could be added to the list, and there would be literally no way a well-intentioned engineer even inside Apple could even know about it. This isn't just a technology that could be applied with technical effort to make a backdoor, it's a deployed backdoor that opens up all our devices to supply chain attacks, plain and simple. A state level actor would simply need to convince someone at NCMEC to insert something into the un-auditable hash list (whose source images are never to be looked at in totality by design), then compromise any person or computer in the law enforcement-side reporting pipeline to exfiltrate the identity of anyone with the images in question. That's absurdly dangerous.