Quod erat demonstrandum does not mean "and thus I have proven beyond all shadow of a doubt that I am right"; it means, roughly, "that which was to be demonstrated". The usage of QED is correct here.
From wikipedia (http://en.wikipedia.org/wiki/Q.E.D.): The phrase is traditionally placed in its abbreviated form at the end of a mathematical proof or philosophical argument when that which was specified in the enunciation, and in the setting-out, has been exactly restated as the conclusion of the demonstration.
I agree with the parent comment that this rhetorical flourish is misleading, and I agree it gives the non-scientific set a hook to riff on.