Luminar lets you add folders from cloud storage locations that are synced to your computer. In this case, synced means that a physical copy of the photo resides on either your internal or external hard drive. If only a reference copy of the photo is located in the cloud storage photo, Luminar will not be able to create a thumbnail or be able to edit the image.
Adding folders from synced cloud storage is very useful in situations where a client or family member adds images remotely to a cloud service server. Once these images are synced locally to your computer, Luminar automatically adds them to your Library.