Sure there is a difference between 50 and 52; that is intended to account for the fact that there is probably a fair difference between what different athletes in that 13:30 range might be able to run.
I am not saying every 2:12 marathoner can run 52 seconds for the 400. I am just emphasizing that those folks who were suggesting 57s for guys who have run 13:30s are being a bit silly. You may have a one off guy here or there that might be in the 53 or 54 range, but I would think that those guys would be the rare exception.
That is borne out by common sense, that is borne out by picking any number of elite marathoners and looking back at their other race times, and that is borne out from my personal observation (both of myself as a 14:19 5000m/55 high 400m at the end of a workout, and by other athletes that I have coached and trained with).