They do pre-board, but from what I've seen, the GA's take "pre-boarding" literally. You have to be on the blue carpet at the moment the GA announces pre-boarding for disabled, military, and GS's, because you have about a 5-10 second window before they start with Group 1. At that point pre-boarding is over and you're now part of groups 1 through whatever the currently active boarding group is at the back of that line.