On a bike you have a little bit of flexibility due to the way the seat post works. Both in how the seat attaches and adding curves to the post, particularly for triathletes who like to favor their hamstrings over their quads, and sit considerably farther forward than any 'normal' cyclist would.
I know I've seen wheelchairs where the back was a tube that went into a tube. If you put the curve in the replaceable part you get more adjustment but less support. Generally the tolerances on bikes are very tight and medical equipment seems to be all over the place.
They've done a lot of work it looks like to put in user level adjustments everywhere they can but to tweak the parameters on the frame page I don't see a great way to replace that. The curve it's tweaking is setting the angle between the legs and the seat already.
Yeah I went through the build your own UI after your last reply and it looks like a pretty complex bracket there for adjusting the angle of the seat back. From the look of things it's an adjustment versus another bracket.
It looks expensive, but I suspect that not being able to change your mind after you order is awful. Anything from muscle gain to tweaking your back probably makes you want to adjust your seat a bit. So maybe it's worth however many dollars that takes out of the $1k budget to be preserved in the design.
The seat back and center of gravity position both look fairly adjustable. The main fixed quantities seem like they're all on the frame page; seat heigh front and back, frame depth (back of seat to knees), front angle, and frame taper. The rest of the options seem pretty adjustable looking at the frame there's a lot of mounting holes to change all the other measurements.
I know I've seen wheelchairs where the back was a tube that went into a tube. If you put the curve in the replaceable part you get more adjustment but less support. Generally the tolerances on bikes are very tight and medical equipment seems to be all over the place.