Interesting. I did read on their forum or something that this additional mesh data can increase the vertex count though. This leads me to believe it may not be reconstructing them, but instead it flips/rotates the weight order based on neighboring triangles, and splits the vertices if there is an impossible scenario. If the alpha does store the index as you say, then it must be reconstructing them though. Maybe I'll look into it one day, but for now this basic approach works for this kind of terrain.