Gregory Maxwell wrote:
Abstract: Wikipedia's brilliance and curse is that any user can edit any of the encyclopedia entries. We introduce the notion of the impact of an edit, measured by the number of times the edited version is viewed.
So I guess editors who delete articles are recorded as not contributing anything to Wikipedia by that action. Whether one sees this as a flaw in the study's methodology or as commentary on editing ethos probably depends on the eye of the beholder. :)
Would have been interesting to see that top ten list of "most impactful" editors they mention in the paper, though that may just be latent editcountitis talking.