I was making a conscious effort to flatten my back when I took those images. I probably don't ride like that in practice, but I've never left my body to witness it in person.
I had to go digging for this photo, but this is my old Soma DCD when I rode Castell Grind in 2016. I think I road the 100K like this.

I am pretty sure I am mashing out of the saddle on a soft, sandy uphill in this photo.