这里为什么用had died?

有一读者问：

I had thought that he had died at least twenty years ago.

这一句中为什么用had died? 它有延续性吗？

这里用过去完成时had died是因为它发生在主句谓语动词had thought之前。虽然that从句可以译作“他至少已死了20年”，但并无延续性。