I'm pretty sure this is one of those situations where both sides of the argument are correct because there are 2 different things going on.
Strictly talking preload, the spring is linear and since the truck's weight still compresses the spring beyond the preload, the total load on the spring is the same. When the truck is parked all you've changed is how much of the shock body extends out from under the bottom spring seat - NOT how much weight is on the spring itself. So nothing has changed there.
But what has changed is the minimum weight to engage the spring. Our trucks have around 9 inches of suspension travel and the standard ride height is about in the middle of that so if you lift about 2.8 inches you have less than 2 inches of additional down travel available, and when you run out of down travel the amount of weight needed to engage the front suspension is increased by whatever the preload weight is.
So if you imagine that the stock preload is 50 lbs and you have 4.5 inches of down-travel, you'll always have that smooth gradient of pressure when one tire dips into a pothole or rolls off the far side of a speed bump and when a tire does fully unload it only takes a 50 lb weight to bring the spring back into the picture, not much.
Now if you preload the spring to 300 lbs by shortening the minimum distance between the top and bottom spring seats & you only have 1.7 inches of down-travel available you will more frequently reach the top of your suspension travel and then when the suspension comes back into engagement it takes 300 lbs for the spring to go from "unloaded" to loaded - that's a much bigger gradient you're going to feel.
