Whilst appearing even handed, in practice the article fails to mention a single reason for Stallman wanting the copyright assignment. I don't know what his reasons are, but he does tend to have thought through these issues carefully.
I think the article author assumes that the fact and the reasons for the FSF insisting on copyright assignment for its software are well known. https://www.gnu.org/licenses/why-assign.en.html has the answer, which basically boils down to "it ensures that there aren't going to be any awkward problems if the FSF ever needs to legally enforce the license."
I think you answered a different question from what GP was asking. The question for most people, which is answered in neither the article nor the email threads in emacs-devel, is why this stuff needs to be "in Emacs". What is wrong with it being a third-party package?
For example, these quotes from Stallman:
> We have a problem in Emacs: it doesn't contain a good interface to
git. People often recommend something that is not in Emacs. That's
not a good situation.
> When people ask here what they should do to use git with Emacs, the
usual answer posted is "use Magit". Thus the problem: that the usual
way people recommend to use Emacs with Git is via a package we have
been unable to include in Emacs.
The obvious question is: what is wrong with using a package that is not in core Emacs?
