what he's saying is that if they could compile the source code themselves, they would be able to take out all the backdoors they could find before doing so. But since they can only read it, not use it, they cannot do that even if they spot something.
Also, there is no way (if they can't compie it themselves) to actually make sure that the copies of windows they are running were actually compiled from the same source code they have been given to read through. Unless they spot some glaring error where one version can do something the other can't...