To see what syscalls nw is using use:
strace -o nw-strace.log -f ./tmp/nwjs/nw .Then look into the nw-strace.log file.
By default, chromium opens a local listening socket at 224.0.0.251:5353 to receive device discovery notifications.
This can be disabled at the chrome://flags/#device-discovery-notifications page (search for Device Discovery Notifications) or with the chromium arg --disable-device-discovery-notifications (like its done with the chromium-args property in package.json).
NB It seems mDNS can only be fully disabled by recompiling the nwjs source; see the enable_mdns build flag at https://github.com/nwjs/chromium.src/blob/6d4b9e7239747d19eb6f26c67dc331fb43727348/net/BUILD.gn#L70.