The Spanish example doesn't seem to prove his point, because there was no effort to charge the existence of a communist-supported insurgency in Spain. All there was is a couple of over-zealous officers. Everyone who was ever in the army participated in exercises. In most exercises, someone played the opposing force.
Training is more realistic done that way.
Second only to the Central Front stand-off in Europe, the US military's most difficult challenge during the Cold War was combating guerrilla wars. And during the last decade of Mr. Prouty's military career, the late 1950s and early 1960s, the focus of the US military was very much on trying to cope with the tactics of guerrilla insurgency. So it should be no small surprise that he personally witnessed exercises in which US forces practiced coping with guerrilla insurgents. It was an exercise. Certainly nothing remotely like that in the newspaper article quoted, or in anything offered here.