EDIT: "Were", not "are", because I've lost touch in the past years and don't know how it is nowadays. Sales have dropped, the audience may have too. (Still see gorgeous women in the bookshops though.)
How do you think sales ratios could provide the figures about people's purchase intentions?
https://www.booksbythefoot.com/shop/pc/Holly-Ivy-3p232.htm
I assume sales to those services are nowhere near as high as 20%, but counting sales to similar services (and Hollywood set designers, bulk orders to interior decorators, etc) could go some way towards calculating a percentage.
As to how they'd know: Editors are the people who effectively decide what books are published. Their income depends on their insight into customer behaviour. Wouldn't you try to learn why books are bought?
Let's consider the problem. Presumably the majority of people who buy books that they have no intention of ever reading would never admit to that fact. How do you then figure out the numbers of such purchases? (Remember that the comment I originally replied to gave specific figures). How does being an editor somehow give you any meaningful insight into those figures?
Also, majority of games bought at steam is never played. People buy it because they are discounted and there never go around playing. It is not for show, cause nobody see that. Large portion of those 1 in four gonna be like that too.