I wouldn't describe it as a "falsehood that programmers believe", just one of those perhaps unfortunate realities that exist for some codebases that have a decent set of unit tests but little else in the way of up-to-date documentation that explains all edge case behaviour etc.
The only documentation you can really trust is that which 100s of others rely on regularly, but a significant percentage of the code most of us work with isn't going to fulfill that criteria.
Explaining edge case behavior is one use-case for comments, and not the most valuable one in my estimation. Aside from that, often incorrect edge case handling is in both the code and the unit test because the problem is that the developer didn't understand the requirements. In my experience, in an undocumented and difficult codebase the tests will be as mysterious or unreliable as the code itself, which makes sense since they are usually written by the same people.
You'd be surprised. There are generations of programmers now who think nothing of writing 20+ unit tests with quite clear names demonstrating what the behaviour should be under a variety of conditions, but with virtually no other documentation. Especially true in dev shops that have high coverage requirements for a successful ci build.
