The point they are making is the tests are neither necessary nor sufficient alone to prove the code does exactly what the spec says. Looking at the tests isn't enough to prove anything; as an extreme example, if no one involved looks at the code, then the tests can just be static always passing and you wouldn't know either way whether or not the code matches the spec or not.

If anyone cared enough they could look at the code and see the problem immediately and with little effort, but we're encouraging a world where no one cares enough to put even that baseline effort because *gestures at* the tests are passing. Who cares how wrong the code is and in what ways if all the lights are green?