I've been using SyncThing [1] recently, which does a similar thing but between your own devices (anything from Android to desktop to servers in the cloud). I've been using it on Linux and Windows, and it seems pretty good.
I don't think Syncthing is the same. As far as I recall, it stores the full sync folder to disk, similar to traditional sync services. This project only stores a cache on the disk.
You are correct. With SyncThing each device gets a full copy of whatever folders you've asked it to sync there. The FUSE aspect of Zero lets it do just-in-time file transfer and have no solid upper limit on storage, saving bandwidth and disk space at the cost of portability, latency, and redundancy.
I built syncthingfuse to do partial syncs. Currently unmaintained, though.