Also, any conscientious coder will put in prominent comments warning others about the trickier parts of the code. Comments like "UGLY TERRIBLE HACK" tend to indicate good code rather than bad: in bad code ugly terrible hacks are considered par for the course.
Despite the above, the quality of the code is generally excellent.
Therefore, a superficial look at the code finds no evidence that Microsoft has violated the GPL or stolen other open source code.
In short, there is nothing really surprising in this leak. Microsoft does not steal open-source code. Their older code is flaky, their modern code excellent. Their programmers are skilled and enthusiastic. Problems are generally due to a trade-off of current quality against vast hardware, software and backward compatibility.