webtorrent builds on webrtc, which requires stun and ice servers. so you still send metadata to defined servers for connection handeling and most sadly you can't p2p in an offline LAN.
This is also true for typical p2p protocols though, isn't it? Normal torrent clients for example have hardcoded servers to start their dht search from.
[0]: https://stackoverflow.com/questions/1332107/how-does-dht-in-...