I'm the opposite, and honestly I always thought too much sitting (combined with too much gym emphasis on the abs and not enough on the back) caused the lumbar curve to flatten or even reverse. After all, when you're seated normally, the pelvis tilts backwards. My lumbar spine was pretty much rigid and vertical before my PT this summer. I can finally get into cobra with my pubic bone on the ground, yay! but it's going to be a while, if ever, before I can keep my ASIS on the ground.

I think part of it is related to having no waist, and probably pelvic anatomy as well. When my pelvis is as neutral as I can get it, I appear to be in extreme swayback. I don't think I could ever sit in a chair in the "tripod" you describe. I just tried it on a bar stool - so, hard flat surface and knees below hips - and forcing my pelvis as far into anterior tilt as I possibly could (straining as hard with my low back muscles as you seem to have been against the wall), I can still get a finger easily between my pubic bone and the seat.