This is supposed to be from the Ptolemaic period. Iron age Greeks, descendants of Alexander the Great’s general.
The "sarcophagus" was found in a tomb supposedly dating from that period, but it's pure conjecture for the the egyptologists to state that the granite box was made by those people.
It's more likely that those iron age people discovered that box and re-purposed it for a burial sarcophagus.
Many of these multi ton stone boxes have been found in Egypt - most containing nothing. Curiously, they're all carved from single blocks of stone, and are finished to degrees of flatness, polish, and mathematical perfection, we can hardly reproduce them with modern technology, today.