There are specialty origami papers (or more commonly, specialty japanese or other asian printmaking papers adopted by origami artists) that are much thinner and tougher than commercially available ones. I do not know where they found 15gsm Kraft since the most common manufacturer used by origami artists, Gascogne Alios Kraft, only makes papers down to 30~ gsm. The lightest paper I've found online was 5~ gsm kozo (a japanese tissue paper) and the lightest I own is 12~ gsm kozo, but I haven't folded it yet. Two well known Western specialty origami paper manufacturers are Paper Circle which makes O-gami, and Origamido. For both, their lightest commercial papers are around 20 gsm but I think they'd try to make lighter ones for custom orders. both papers are incredibly durable and feel almost miraculous to fold if you're used to just crappy 6 inch kami/ "origami paper".

Note that density (weight/area) does not perfectly correlate with thickness but it's usually used as shorthand.