How to simulate collisions on leaning trees?

From what I can tell, terrain engine trees support only a single capsule collider per tree instance. Worse yet, they don't allow that collider to be tilted.

Is this really the case?

How are we supposed to use trees which are leaning, or include multiple trunks?

I tried creating children capsule colliders in my tree prefab, but the terrain engine simply ignores them.

For these types of trees, you will have to use a regular scene prefab. You will not be able to place a large number of these special trees in your scene, and they will not be affected by wind etc. unless you write some scripts.