When people say reverse proxy they usually mean using nginx/etc to forward traffic to a normal service listening on a port, in that case you still need an open port. In Tunnelmole your "server" establishes long lived outbound connections to another server which forwards traffic to you that way. This technique is commonly used by penetration testers to use "reverse shells" on compromised boxes. It is a very handy technique because it bypasses NATs and inbound firewalls, however it does not scale as well as a traditional reverse proxy.