Syncthing is a file sharing server and clients can be used to download files from it, or upload them, AFAIK?
ZeroTier establishes the connection only which can be then used for whatever you want. Ah okay got it that it is not server <> client based like a VPN. But it as well allows several peers to be connected in a network, if I read correctly?
And even if it is not client <> server based, it requires the "root servers" to DNS-alike locate target peers. I think the main benefit over classic network connections, VPN and such is that peers can always connect directly wherever they are. Very interesting for mobile handled devices which connect from different locations through different IPs. Indeed in case of VPN this can be only done when the two peers are VPN clients and having everything relayed through the VPN server which requires a static location/IP/domain.
Okay the picture gets clearer. Indeed an interesting approach.
Suggestions for features and software you would like to see in DietPi, goes here.
I was thinking that SyncThing and ZeroTeir have a similar underlying way of operating if having diffeent uses. Maybe I'm wrong. I tried ZeroTier manually but got a bit lost - I've shelved it for the time being.