Toward a "modern" Emacs

No, they have not, this is correct. It is not possible to assign your copyrights to another party and then for them to 'assign it back to you'. If that were possible, the net effect of that document would be zero.

You assign your copyrights to the FSF, and they grant you a broad license (beyond the terms of the GPL or any other FSF software license) to continue making use of your contributions in any way you wish (as long as that use does not conflict with the FSF's use of the same contributions).