Why wouldn't the cal impose a different mounting position though? As the suspension rotates, the top of the cal track mount hits the leaf, at which point it behaves like a solid piece of steel mounted at the spring hole position, and the position where it hits the leaf,more motion is constrained by the interference with the leaf.

As a result, it acts as though it were a solid piece, which should allow it to behave like a geometry defining link. It's motion is more complex though, as it relies on the leaf to present the mounting point for the bracket, and while it clearly hits the leaf and thus defines a new link, the leaf still has some movement, and as a result the ic isn't as solidly defined as it would be if the cal track mount were actually welded to the frame.
(not saying that should be done, just pointing out how the motion of the bracket, though constrained, isn't quite fixed. Also, this is an order of magnitude thing, the motion is hardly unlivable. After all, the chassis is trying to squat, so the leaf is trying to lengthen, which means the leaf is pushing against the bracket, not pulling away from it.)