I have made a bunch of PVC bows as a kind of bow-making experiments and testing design principles in a more easy way than with wood, taking things to extremes.
As a result of this, theses bows very often failed at some point. When doing a post-mortem analysis, and cutting up the pipes, I could see that the most bending portions, which took the most strain, were often fretted all along the inside of the pipe's back along the entire hollow area (which is similar to the concave belly of the HLD bows).
So maybe the HLD is indeed a way to divide the stress over a broader belly area, even curved.