I still don't get it. There is no "preload". The weight of the vehicle is on the coil, so the coil is loaded. I guess you could call that its "preload" since there would be more loads to come as he drives. It would be loaded more (dynamically) as the suspension is compressed during driving. But, it's just loaded nonetheless. With the weight of that corner of the vehicle on the coil, it will compress a certain amount until the spring rate and the weight acting on it balance out and that's it.
Here's my angle:
If I were to hold my hand out and set a lightweight spring in it (think wastegate spring), then set a 5 lb book on top of that spring but kept my hand perfectly still, the spring would compress X amount. If I raised my hand, you're telling me that the spring would compress (assume more preload) while the book would remain at or very near the same height in relation to the ground?
I bet that if Dan cranked up on the perch as far as he could without hitting the maximum extension of the shock and measured the coil's overall height, then lowered the perch as far as he could without the shock hitting its internal bump stop or any other suspension component hitting whatever limits its movement and measured the coil's overall height, the two measurements would be the same, within a tiny fraction. There would be a little weight change acting on the spring due to the shifted weight of the vehicle.