There's another paper describing the "Hogbatch" approach that shows more exactly the effect of adding cores on accuracy: http://www.ece.ubc.ca/~matei/papers/ipdps16.pdf.

The summary would be that accuracy per pass suffers slightly, but since the speedup is close to linear for the first dozen or so cores, each pass is much faster to run. The result is that the wall time to achieve a given level of accuracy is much shorter despite the slightly lower accuracy per pass.