> I realize that the stiffness of the rockers and links will make a difference in how forces are distributed, because that suspension is clearly not statically determinant

I think it's (mostly) the terrain geometry (plus gravity) that constrains the system, not joint or bar stiffness. This makes them feasible to analyze with the tools of theory of mechanisms I believe.

Also a really noteworthy insight I think he had was that when there is some kind of non-planar geometry arrangement of the wheels, that tends to create additional (perpendicular to surface) force on the wheels, effectively increasing the coefficient of friction. Think how a child may be able to climb a door frame by pressing hard enough on the sides.