Source on that? While Emacs has pretty good mouse support these days, that wasn't always the case. I'd be surprised if pre-19 versions were mouseable at all.
Anyone not using a space-cadet or Symbolics keyboard could have remapped the mouse buttons to Ctrl and Meta to avoid Emacs pinky, but I still find it hard to believe that GNUmacs prior to the rise of Lucid/XEmacs had any real mouse support at all.
