Isn't that a point of commonality with Lisp Machines? Not that I've used one, but from what I read, one of their great aspects is that every function of everything running in a Lisp OS was available for impromptu runtime modification. IOW, you could modify anything even while it's running, without the need to rebuild it and restart it like we do now.
Having a global namespace makes the hot-reloading easier by not having to figure out in what namespace the redefinition should be happening.
That feature is pretty much only widely available in Emacs, now.
It certainly sounds magical. And the few times I work with bugs in Emacs libraries, it is amazing to jump to source so effectively.