@Jetah i don't think they will be hard to build, because i am sure, Michele and his Art Team knows well how to use 3D programs to get the needed assets. That will be their main job at the Studio. So i guess they can create in no time the assets they need. Maybe the more complex things will take a while, but to make Trees, Plants, Logs and and Stones are the things every Game Graphic Designer need to know as a basic.
Even me, who is an absolutely Noob at Blender, will be able to create these small assets - probably with less nice texture, but still.
To accuse them, that they mainly used built-in and cheap packs assets, is something you have to prove.
I was today searching in the Asset Database of Unity and i did not found any similar of them.
But maybe the OP has some evidence.