The dynastic Egyptians didn't have tools hard enough to carve granite.
I would suggest that box was carved by some as yet unknown, pre-dynastic culture, who possesed the technology to fashion it.
This is supposed to be from the Ptolemaic period. Iron age Greeks, descendants of Alexander the Great’s general.
They certainly had the tools to shape granite... all one needs to do that is use hammerstones. It’s laborious but American Indians worked granite into discoidals, celts and spuds and the like the same way, albeit on a smaller scale.
black granite is interesting, as it’s supposed to be the least porous granite. I’ve read King Tut’s daddy’s sarcophagus was made of a single piece of Chinese alabaster. One has to wonder where the black granite (India? South Africa? Zimbabwe?) and the alabaster (China?) came from?
Black granite is a lot heavier (more dense)and harder than the standard Barrie Grey. This sarcophagus would have been a monster job for whoever fashioned it. The black granite I work with rings with a high clear tone when struck with a rubber mallet.
The "real" Egyptians were an ancient memory by the time of this burial. This is post-Alexander, when Egypt and the rest of the eastern Mediterranean were ruled by Greek successor kingdoms. At the later end of the dating period, it's at the time of Actium, when Egypt was about to become a Roman province.