What's psychosocial reductionism?

Allen Frances has clarified his position on psychiatric diagnosis in his Saving Normal series on Psychology Today (see article). Although he's been critical of DSM-5, in response to Lucy Johnstone, he makes clear that, for him, the biological model is essential and that neuroscience has been "enormously successful in helping us understand normal brain functioning and that over time, in very small steps, this will result in better understanding of abnormal brain functioning". However, he doesn't explain what he means by this.

And he doesn't seem to understand the meaning of reductionism. Reducing the psychosocial to physiological processes is what psychiatry does all the time. I'm not sure how the psychosocial can be reduced to the psychosocial. Presumably Frances is suggesting that the psychosocial is a simpler form of something more complex, but is it?