I already said REE (the operator) was partly to blame, but they were reacting to something their grid was ill equipped to deal with.

Perhaps you should understand the difference between distal and proximal causes of events? Both are important. Voltage oscillation was the proximal cause. Where did this come from? It's explained in the operators own report:

During the incident analysis, it was determined that the oscillation was not natural to the system but rather forced. This oscillation is observed with significant amplitude at a Photovoltaic Plant located in province of Badajoz (PV Plant A). At the time of the oscillations, the plant was generating approximately 250 MW. Since the oscillation was forced, it ceased once the plant stabilizes it.