ICs aren't fixed in motion, that's why they're instant centers. IC can be derived using devices which only operate in a certain suspension mode. Decoupled torque arms come to mind. IC doesn't define a solid point regardless of motion, it only defines the point at some particular instant.
IOW, at the instant the Cal Tracs engage IC can be derived by the system for just the point at which they are engaged.

A/S (Which is what most look for by moving IC) Can be exactly 100%, which completely binds the system, no suspension movement at all. If it relied on the motion of pivot points to work, than the bound suspension wouldn't have IC at all. That is what occurs under A/S after all, it's the chassis exerting force through suspension geometry instead of through the springs. It happens before the suspension moves.

If you've done front suspension you'd know it as Roll Center. But whereas a roll center near to cg on is undesirable, it's used on the rear as a/s because:
If there is a rate of change in acceleration (Not just 1g, but going from 0 to 1g) some of the force applied through the links can actually be exerted downward on the tires. That's why A/S provides that initial bite. With a large rate of change in acceleration there is also a large downward force on the tires. The trade off being that suspension compliance under accel goes out the window, as the system is in bind.

And again, the leaf only applies force through one point, but with it bound as it is by the Cal trac stopper it makes an excellent place mount another momentary pivot, Which is the cal trac.