ZeroTier is more a Software Defined Network than a VPN. It does not force all traffic to go through it unless YOU configure it to do so. It’s not configured like that out of the box. It’s more like putting a second Ethernet card in your machine plugging it into a second LAN. Via that adapter, you can only talk to other things also plugged into the second LAN. From there, if you so choose, you can limit traffic to only SSH via the rules engine, or by configuring the firewalls on the machines connected to the network to only allow incoming SSH connections.