Not really true. Not forwarding and/or blocking incoming ports just prevents unrequested incoming traffic. Unless you block/filter/isolate it in some way, then if it's on your network it can generate valid outgoing and two-way traffic in response all that it wants to just like any other local machine on your net. That includes simple annoying stuff like phoning home through at least potentially things like setting up tunnels, capturing local network traffic and forwarding it out, downloading and executing malware, attacks against remote and other local computers, altering its own firmware, etc., etc.