I spent about ten minutes on the site, so hardly a domain expert. I’m still confused, too. But as best I can understand, you have the option of storage on S3, Azure, and the like. I assume that with a plug-in/driver, you could store anywhere you like.

But non-local storage does seem to be designed in, because there is text like “if there’s a daemon running rsync in the background, you’re doing it wrong” and “if your UI requires marking folders to be synched/not synched, it’s broken”, so there appears to be an assumption of putting your data elsewhere.