Dropbox uses phone APIs that were designed to do exactly this. There are not "cloud storage" APIs that simulate a file system in desktop OSes. So Dropbox had to

Dropbox uses phone APIs that were designed to do exactly this. There are not "cloud storage" APIs that simulate a file system in desktop OSes. So Dropbox had to implement the syncing logic. And they also, now, have to implement the cloud file system logic to get this new feature.