I don't think it's about the language - it's about knowledge. A backend specialist can learn Javascript but may not know the nuances of working with the DOM, or optimizing for pagespeed, or the important front end libraries. Similarly, a front-end specialist may not have all the experience necessary to diagnose problems in a deployment. That's one reason I don't buy the "front end developers can now do backend development because of node.js" argument. Any sufficient competent backend developer can start writing Python code in a day. What that developer doesn't get in a day is understanding deployments, popular Python libraries, the ecosystem, etc.