It is not expressly required to use node on the server side. As an example, Flask has as server-side library using python:
https://flask-socketio.readthedocs.io/en/latest/
socket.io also has a C server library, and I am sure there are a number of other languages that can use it. The issue here is that all the node documentation uses the "/socket.io/socket.io.js"script URL which implies that node.js is supplying the file somehow but we would like to get that file to serve without using Node.
In the flask example, it looks like they serve it using this script tag:
<script src="//cdnjs.cloudflare.com/ajax/libs/socket.io/2.2.0/socket.io.js" integrity="sha256-yr4fRk/GU1ehYJPAs8P4JlTgu0Hdsp4ZKrx8bDEDC3I=" crossorigin="anonymous"></script>
Depending on what your Server-side is using, they may have their own client-side implementation as well. I personally recommend reading the documentation for your socket.io implementation to see if they specify a client library.