And furthermore it came as news enough to other researchers that the paper got cited many times.

But rediscovering some old math doesn't make it that bad. I've rediscovered old math before, and it was something that mathematicians around me thought was interesting because they hadn't seen it before either. What makes this particularly egregious is that everyone involved theoretically took a course that not only described this exact technique, but which described an even better one! (Simpson's rule.)