The short answer to your first question is “yes,” and the short answer to the second question is “no” – you could make a packraft with only flat seams, but that would be stretching the definition of “packraft”… something like the Klymit LiteWater Dinghy is about as close as you can get.
That Anfibio packraft you linked to has a flat seam around the outside, but the cockpit seams are not flat. I made a prototype like that a few years ago and when it was done I didn’t think it was worth pursuing any further.