The answer depends a bit on whether you are going to ingest in Classic or ingest in Cloudy's Lightroom.
If you ingest in Classic only the smart previews are in the cloud, so no, it is not a backup. I'm not sure if CC could export at all then, but the loss of detail would be large if so.
If you ingest in Cloudy, then originals are stored in the cloud. But I think there are a lot of issues one who is appropriately paranoid needs to think through in terms of backup there, notably what is an "original".
I want to have a backup of my "original" meaning as close to what I produce from the camera, or from Photoshop edits, as I can get. The more programs that manipulate it (e.g. copying it from your system to the cloud, storing it, moving it back down later to edit again) the less it is truly "original" and the more possible it is that bugs in those programs affected it.
With Cloudy you can store originals locally, but if you ingest or edit on multiple computers, there is no good way (or single point) to back those up before Adobe runs it through their processes. To me that is an issue, as we have no visibility into Adobe's system, how redundant it is, what kind of integrity checks are in transmissions, storage or processing. The basic premise is "Trust Adobe".
There is some good reason to trust adobe. They are more reliable than your average home system or average home user (considering your average home user apparently feels no need for backups at all, so welcome to the club of us not-normal people
)
But if you work in technology long enough you learn that the best bet is not to trust anyone. Emphasis on "one". Redundancy and independence is always best.
I love Classic in that sense. I know exactly where the originals come from, and are stored. I can back them up in any way I want, and check them for integrity in various ways, and have complete control.
Anyway to your question, I think telling us more about your workflow would help to understand to what extent the cloudy copy is usable as a last ditch backup.