Because someone understood and verified it.
And that some one can be an LLM. Does your manager at your job review every line of code you wrote or trust you based on your competence and reviews only a compressed portion?
Frankly, these discussions are tiring because there's a false notion that humans have to be in the loop 100% of the time. This is untrue. LLMs are accurate enough that they can be left to do things on their own and human intervention is needed only for out of distribution instances.
Didn't the Postgres team just have to stop a release because the agents found a slew of bugs in human code from domain experts that "understood and verified it"?
Perhaps I'm being overly generous to the parent's point, but it stands to reason that we make a lot of choices of where and how much trust we give away. This is just another one of those points.