Yeah, Douglas is very quotable, but his quotable lines tend to be broad generalisations. The lines between serial, mass and spree killers aren't hard lines, people are too chaotic for that. There's a lot of blurring and overlap.
For example, family annihilators that go on the run like List, Fisher, and de Ligonnes not only got away with it, they did so incredibly successfully, because they all planned meticulously. They are, by definition, mass murderers.
While the perpetrator of this crime is by definition a mass murderer, in my opinion, he has more in common with the general pattern of an organised serial killer. He planned to get away with it. That says more about him than how many times he did it.
MOO