Suppose this was not Emacs. Suppose this was KDE (copyleft, no copyright assignment) or Xorg (non-copyleft).
A contributor commits code to one of the version-control repositories of those projects. As soon as this was done, the code is already published. He can't (legally) retract the code that was published (Right? AINAL). You don't need any extra permission from the author to use that code.