If, by your definition of understanding, you can understand something without knowing how it works, then what would be the point in understanding it?
Your argument underscores the fact that understanding exists on a spectrum and that deep understanding can be difficult, or even impossible to achieve. Even formal verification seeks to verify certain properties of a system, and doesn't represent anything close to comprehensive understanding, and the process of formal verification is also guided by a fallible human. In a lot of ways, formal verification is just pushing up to review of a different "programming language."
So, good review is really hard, but ignoring the difficulty doesn't make it go away.