You are right about that, but having code that passes a given test suite doesn't say anything about its secondary qualities, such as whether it can be understood. In theory, a failing test could improve your understanding of the situation, allowing you to refactor your initial pass at a solution, but I would bet that on this particular code base, the comprehension-raising doesn't go far enough, in most cases, for this to be feasible.
That seems orthogonal to testing though. Implementation code can be hard to understand with or without a test suite, at least with test, as you point out, you may be able to understand the behaviour at some higher abstraction.
