The JS spec says nothing about threads or coroutines, just like the C spec says nothing about them either. That doesn't mean that they are incompatible with the language. In fact, real multi-threaded JS code runs just fine on the JVM for example.

It works fine with the latest Node for me. Fibers relies on libcoro which has only had minor updates over the past four years.

I'm not sure where you're seeing this code sharing. In practice the only code that is shared is utility methods that don't do any I/O, so whether you use fibers or not makes no difference.

In fact, Meteor, being the framework that is pushing the boundaries of code sharing between client and server, uses fibers on the server.