You state it uses PeerJS. And indeed, the code calls from PeerJS,

  const peer = new Peer();

I.e., it constructs a PeerJS Peer without manually specifying PeerOptions. That class, in turn, states,

  /**
   * Configuration hash passed to RTCPeerConnection.
   * This hash contains any custom ICE/TURN server configuration.
   *
   * Defaults to {@apilink util.defaultConfig}
   */
  config?: any;

… so it sure looks to me like it is using a TURN server…