How's this an error? The author clearly pointed out that the immigrants themselves will cost more on average, while the descendants will be beneficial. Since the descendants will live here, why shouldn't they be considered when looking at the impact of immigration.
Gillespie states the following:
Furthermore, a typical newcomer pays $80,000 more in taxes than he takes out in benefits over the course of a lifetime.
Gillespie doesn't mention any descendants. He uses the immigrant + descendant figure to describe the immigrant figure alone. He couldn't use the overall immigrant figure because it was $-3000. It would have been weak to describe "immigrants and their descendants", because descendant tax revenue is a pig in a poke. So he just slipped in the $80,000 figure.