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.
He uses the term 'newcomer' to describe the people who would be here with immigration and wouldn't be here without it. That figure is the one that matters. It makes absolutely no sense not to look at the descendants, because clearly they'll live here.
It would have been weak to describe "immigrants and their descendants", because descendant tax revenue is a pig in a poke.
No more so than it is for immigrants. That is, you are assuming that the previous trends will hold for both groups. But why does it make sense to do so for immigrants but not their children?