Even if they think their math is sound, based on how I interpret this it still could result in elites sitting in the back while buyups are up front. Consider the following two scenarios:
1) 5 elites eligible for upgrade, 6 available seats. They sell one of these FC upgrades with a confirmed seat assignment, leaving 5 elites for 5 seats. At the T-1 mark, a Plat elite adds a companion, which slides in ahead of the silver that was #5 on the list. The silver who was #5 does not get the battlefield upgrade, while there is a buyup up front
2) Same scenario, but instead of a companion upgrade, they sell a few revenue seats at the last minute. Again, a buyup was confirmed up front at check-in when they thought they had fewer elites than seats, but now because of revenue sales the bottom elites on the list do not clear at the gate.