yeah, sounds to me like their using Gamebryo as a starting point for the new engine and not necessary a slightly upgrade version. i can imagine that it has to do with how the game handles the open world environment since there were few issues in that regard with either Fallout 3 or New Vegas. everything else with that engine needs a significant overhaul to keep up with new engines coming from iD and Crytek (particularly Crytek given that both previous Crytek engines handled large open environments very well).
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind