I dont know why this would be specific to folding bikes, but any engineer or construction worker will tell you that the strongest shape is one where the edges have the farthest average distance from the center. This means that a circle, where all the edges are the same distance, is the stongest. Triangles are also very strong, being used primarily in bridges, and you can see (two) triangles on bikes, which is where the strength in several directions comes from. I-beams are decently strong, but can only take stress in several simple directions, which is why they are used in building, because in a building, the building can only sway in 2 ways (hopefully!) and doesnt need anything special. Because of the strength (equadistant edges) to weight (hollow) ratio, tubes are very good to use on bikes.

I dont know enough about carbon to comment on monocoque.

Peter