You don't write the compiler, of course, but you do write or rather design the types that your program uses. The typechecker ensures that they're consistent within the system of logic that they set up.
By your logic, it's not enough to have faith in unit tests because you wrote them; you must also write the test framework and runner.
By your logic, it's not enough to have faith in unit tests because you wrote them; you must also write the test framework and runner.