Yeah, first time I looked at the seat design I thought "That lattice structure has the same amount of aluminum as a solid sheet."
Maybe it conforms better, being separate pieces that can pivot a little at the rivets.
The plans seat is theoretically very strong, but if it started to fail in a crash, say due to uneven loading, it seems like to could cut you to pieces like a cheese grater.
For comparison, at 36"x18" the plans seat is half a square yard, while something like 1000D Cordura nylon w/DWR coating weighs 11oz/sqyd.