This seems incredibly interesting, but the idea of a 'general purpose syncing service', in the way he describes it, makes my head scream'security concern'. In general the way it's described the format is not fixed for these services so your data might as well be encrypted in any arbitrary way I think?
But knowing this wouldn't this kind of general purpose syncing service need some way of identifying what data it is even syncing? Unless you encooperate something grand like the signal protocol (as in encrypted anonymous messaging) you d always run a security risk if the service you use for syncing is not self-controlled?
If anyone has more insight on this I'd be very interested, it seems like a very good concept.
It sounds to me like anything other than p2p local syncing with some protocol is a confidentiality no-go.