I'm doing a home conversion build as well and have found the same, pouch cells may be the best but sourcing high quality pouches and materials to build a good pack with them is far more difficult than with cylindrical cells. There's not an equivalent to say, IMRbatteries.com that sells Farasis 32ah cells.
There's also the fact that cylindrical cells are just smaller, so you can arrange them to fit odd shapes and sizes. In my build, I can fit 4.3kwh of cylindrical cells where I can barely fit 2kwh of pouch cells. If we're looking at volume alone the pouches are significantly denser and pack more efficiently, but if you don't have a perfect squared off shape the benefit of that density could be completely negated.
Which is what we have here with the curtiss. The bike was made from a drawing, it's a piece of art over anything else. And using pouch cells in that cylinder shaped battery pack would be an incredibly inefficient use of space that would have far less density than going with cylindrical cells. It doesn't make sense to equate them using 21700's with not being able to afford pouch cells, when it's obviously a function of fitting as much battery as possible into the design and not a question of cost.
If they wanted the most efficient battery choice they would have put pouches in a big rectangular box, but obviously that is not the point of this bike.