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.
This assumes the sarcophagus was made in the Ptolemaic era. What if the black granite box itself is much older? A test on the mortar used to seal the sarcophagus would be enlightening..
Yes, I understand that. My entire focus is on the granite box. I contend that it was produced by someone other than the ancient Greeks or Egyptians. Neither of them had the tools or technology to cut and machine granite.