CTO of Safing here. I hope I can bring some clarity into this.
Portmaster is actually a privacy suite consisting of many features and modules. It is often described as an "application firewall" to give people a quick, but incomplete idea of what it is.
The SPN is one of these features. It is a blend of VPN and Tor - oversimplified - and is fully optional. In fact, it is a paid feature and won't activate without logging in with an eligible account.