Abstraction in photography had its roots in modernism. We can think of László Moholgy-Nagy, Aaron Siskind, Man Ray, Paul Outerbridge, Barbara Kasten amongst others.
However, he welcomed the literal in photography. In his essay 'Four Photographers' he says:
The art in photography is literary art before it is anything else: its triumphs and monuments are historical, antecdotal, reportorial onservsational before they are purely pictorial...The photograph has a story to tell if it is to work as art.
In Greenberg's view the medium of photography was transparent and documentary. The inference is that photography was not a part of abstract modernist movement. Photography was seen to be elbowing into the modernist art context of abstract expressionism.
Today, ironically, it is photography's exploration of the literary, or interpretating literary themes, (eg. Jeff Wall) that is celebrated by an art institutionthat links it to earlier developments in painting. Recent large scale museum photography---Andreas Gursky, Thomas Struth, Jeff Wall, and Thomas Demand--- fits tightly into the history of painting and the pictorial tradition.