Yes, two of the Prefabs (Coral_B and Grass) currently have disconnected materials. This has been fixed in version 1.3 (out soon).
In the meantime, you can quickly fix this yourself by dragging the Coral_B_mtl and Grass_mtl from the FBX/Materials folder onto the Prefab in the display window to reconnect it. Don’t forget to drag the fixed prefab on top of old prefab in the Project window to save the changers.
Beware! I suspect this is no longer on the asset store for a reason!
I also bought some of their assets. I especially liked their underwater explosion. Until I saw a video of the Slow Mo Guys on YouTube and realized Rivermill had clearly taken it from the video. I confronted them with those screenshots and asked them if they had the rights to use the footage. They never replied.
Years later, someone I know who frequently searches the asset store for copyright violations reported this to Unity. As a result, their assets were removed from the store.
If someone is actually using this material. Remember that there is a high probability that you will not be allowed to use it.