The thing about the accuracy, is that it is being helped a lot by the quarterly reports we have from Sony and Nintendo, helping a lot the calculations over the years (and Will said himself that his calculations are also based on historical figures).
The quarterly numbers are the biggest help for adjustments as we generally know how the usual gap between shipped vs sold. Though, right now that gap is far smaller than usual due to the shortages. Even Switch stock is more limited than what is typical.
One way I use historical data is looking at how much above or below PS5 and Xbox Series X|S sales are compared to PS4 and Xbox One. And for Switch how well sales have been compared to last year, 2 years ago, etc. Of course I also use all available data.