The article also reminds people that for basically the first half of the open era the top guys only played 3 slams a year. Most seem to conveniently forget that little tidbit.
15-3 = 12 which is Borg +1, which is accounted for by the French title.
If you compare Borg's performance on clay with Federer's on Hard, and Borg and Fed at Wimbly, and Fed's performance on Clay with Borg's on Hard, I think you get the best comparison. Fed is stronger at Wimbly and weakest surface is better then Borg's Hard. His best surface is not quite as strong as Borg, but he'll pass Borg if he wins the USO this year.