The detail that gets me is the Ministry saying the cameras were safe because they'd be on a closed network, while the backdoor can apparently be triggered via SMS from hardcoded phone numbers.

That's a pretty good demonstration of why "it's not exposed to the internet" isn't a security boundary if the device itself has an out-of-band communications path.

Add Secure Boot being disabled and unauthenticated live streams and this seems less like one unfortunate backdoor and more like nobody established what the trust boundary was supposed to be in the first place.