I think that when they say "the complete 1790 U.S. Federal Census" they mean "all of the 1790 census that currently exists."
Ancestry.com also says this re: the 1790 Census: "...we have about two-thirds of the original census from the time period. The 1790 census suffered district losses of Delaware, Georgia, Kentucky, New Jersey, and Virginia. However, some of the schedules for these states have been re-created using tax lists and other records. Virginia was eventually reconstructed from tax lists as well as some counties from North Carolina and Maryland."