I wonder how it's possible to get an effective hull shape using a folded sheet.

No expert and since everyone is talking around and not at your question I will take a stab. I would say it is pretty hard.

One thing I learned designing boat hulls is that everything is compromise. If you want high stability it it typically means your going to have higher resistance. You need a shorter boat, that you give up higher speed potential. You want to race, longer less stable boats. Everything is give and take so you have to figure out where your priorities are and be willing to live compromise.

I think it is safe to say their goals were a folding boat and an efficient hull was secondary. Not to say it is bad hull shape but it most certainly a compromise. Working with flat panels that will fold up has to but sever limits on your design.

But, I suspect for those that are interested it gets them on the water and that is their priority. When used in conditions it is designed for and by the intended users it probably works OK. And while I admire the ingenuity is sure isn't my cup of tea.