Planting in poor draining soil

[Sovreali]

I want to start planting fruit trees and already purchased two citrus trees. But I've discovered the soil here is not well draining, I'm guessing a lot of clay. I spoke to a farmer at the farmers market and he told me to do the following : make a really deep and wide hole, excavate a 4x4x4' area, for each tree, then shovel the soil back in and plant the tree. Then afterward put a layer of compost on top to start improving the soil.

Why would this work? I'm a little confused because it seems like I'm just taking the clay soil out and putting the same clay soil back in. Is the point of digging a big hole like this just to loosen up the soil in the whole general area so the roots of the tree can spread out more easily?

But once I start watering the tree won't it still be the same poor draining soil leading to the same problems?