I did not make the game you are asking about, but WebRTC has an even better feature than TCP, which is unreliable datagrams (essentially UDP). Very few people have done anything with it, since server support for WebRTC data channels is very limited. Still, you can use dirty hacks such as leveraging Puppeteer as a WebRTC endpoint and make true lag-free experiences for the web.

Last I checked a few months ago this library was working for WebRTC data channels on node.js. Check it out, but it might not work properly:

https://github.com/node-webrtc/node-webrtc