After reading a little bit more, it seems he includes transfers coming in. So their production is added to both the numerator and denominator (meaning it can only help)

Last year Charlie Brewer threw for 2k yards.

So that means that Utah QBs considered here had the following yardage last year
Brewer: 1958
Bentley: 882
Lisk: 152
Rising: 45

So they're returning 2003/3037 passing yards or 66% of their pass yards, which raises them up a fair amount. They probably also get similar (but smaller - both in quantity and in importance with how little RB is weighted) bonuses from Chris Curry and other transfers