Here's the pertinent graph from the consulation that shows even the highest attaining non-selective schools have fewer affluent families than grammars. The majority of pupils in selective schools come from affluent families.
All that means is the highest tax payers are getting 'quite' rightly access to the best schools for their children !
These families have paid via taxation, why should they not have greater representation in the better schools.
Also above average, below average incomes does not determine where affluence or advantage begin there is no magic line to be crossed in ether direction . The designation of above or below average incomes and is no more than a marker ,it is certainly not definitive.