This isn't a "study", it's a statistical model. It reflects the assumptions put into it. To wit: > Our analysis follows four key steps. First, for all locations where weekly or monthly all-cause mortality has been reported since the start of the pandemic, we estimate how much mortality increased compared to the expected death rate....Second, based on a range of studies and consideration of other evidence, we estimate…
Also: The author cited the sources for their assumptions, specifically on COVID mortality classification.