User blog comment:IamJakuhoRaikoben/Chapter 412: Dance Unto Ishgar, Review./@comment-1359322-20141208144403/@comment-9595671-20141208164830

Recall how Acnologia's roar attack was thought to have vaporized Tenrou Island? That's why its possible that some dragons rotted to feath but many more were simply vaporized by Acnologia's roar.

And while the graveyard is big Dragons were once said to rule the FT world, which means that the whole continent and even the entire world were inhabited by them at one point, that graveyard could easily just be one of many dragon graveyards.