I think it can be optimized a lot further. Some of it would be in the engine and some of it would be in the game projects. I would start calling this 'well optimised' if it can achieve about 1.5-2x the performance of what it is now. (So with 50-60fps on 1080p ultra on my gtx1060, I would be very pleased.)
As far as occlusion; The game does use Godots built in occlusion culling (But I don't think its really that effective with all these alpha clipped materials), but more important it uses frustum culling with the vegetation in chunks. (Possible thanks to ProtonScatter)
The demo utilizes billboards (impostors) for the trees in the distance. For better performance I could have used a better setup for LODs in general.