If you don't store the originals locally using that option, I don't believe there is a way for you to backup your entire online "catalog". It is on the Adobe servers.
I would use the 'storing originals locally' option but there is a problem connected to it. I don't have enough space on my laptop to store all photos on it. That is one of the reasons why I went for the Lightroom CC cloud option: to free up space on the laptop. I would still like to have a local, non-cloud backup of my photos. So I tried storing my originals on an external drive using the tick box in Lightroom CC. Then, however, when the drive is not connected to my laptop and I want to edit photos on it, Lightroom CC does not load the 'on the fly' originals from the cloud, which is unfortunate. So that option does not work for me.
But I might have found a solution for my problem: I guess I could simply use the Adobe Downloader app. I will try this at some point in the next week and then update this thread. It might help others.
Here is the link to the downloader app: Download synced Lightroom photos and videos from the cloud to a computer
Cheers!