The Longhorn project team originally wanted to produce a big flashy OS (see the YouTube videos on the previous page) with lots of innovative (by Microsoft standards) features and stable technology under the hood. But, like all project developers, their eyes were bigger than their stomachs. As is usual with IT projects, featuritis, focus creep, and deadline issue caused the original vision to be drastically scaled down. Longhorn (as defined by this project) was the concept, Vista was the actual product.
So, it depends on whether you think of Longhorn the initial concept, or Longhorn the name of Vista beta releases. I guess after watching those videos, it became clear to me that this project was more interested in the former.