Hey there glad you like it! To be honest no idea, it should work because as far as I know Unity Terrain Brush Tool works with most models, maybe it's changing the textures or materials in some way?
Seems to be only using up the bark part of the model, since the top part is a child object of the bark.
Oh yeah, I assume Unity Terrain only takes the parent model which is the trunk, to fix that you would need to join the meshes together
