I’m only a pilot, but I presume that the question on the engineering community's mind is why there are two offset drill holes in this particular location which have removed so much material (say 15% of the width) from the horizontal leg of the lower spar cap. The drill work does not look very skillful in my inexpert opinion.
Again IMHO, I venture to speculate that fatigue induced failure (whether corrosion related or not) would seem to have been an inevitable consequence and just a matter of time and cycles, once these offset drill holes were made.