The artist was not “foregoing” the realism of Western art, as he was likely unfamiliar with Western (European and Ancient Greek and Roman) art contemporary to, or earlier than, his time, the 13th century C.E China. What would be more interesting would be to compare this simple composition with other Chinese art from the period, and from prior periods. Much Chinese painting eschewed background settings, as can be seen in this work, A Man Herding Horses, from the Tang Dynasty (700s C.E.)

https://en.wikipedia.org/wiki/Chinese_art#/media/File:Palefrenier_menant_deux_chevaux_par_Han_Gan.jpg

What does a Chinese or Japanese art historian have to say about this work? It is not enough to say that Cahill’s appreciation was incomplete.