Indeed, it's the conditions of "selling into strength" that I'm questioning with respect to the anecdotal evidence presented by the current GDX trade. I am not sure what may constitute a better exit condition, merely speculating that somehow factoring the-then prevailing forward-LT/ST stats may prove useful in testing.
As a side note, Figure 3 in the GDX doc shows the vast portion of return (GDX equity curve on a log-Y scale) to be produced between Aug 2008 and April 2009, a rather narrow and unique period of time, which may make judgement of the rest of the ~4yr test period somewhat challenging.