What emperor? Egypt was a Persian province prior to Alexander’s conquest.
nectanebo II’s sarcophagus - you will have read the extrapolation of size of funerary block from the relics in venice as well. this sarcophagus was referred to as having been used for alexander well into recorded history.
nectanebo certainly never got to use it, and ptolemy would have needed something suitable fast while the funerary casing was being made.
one of those interesting sets of suggestive data points which we are very unlikely to ever get anything further on.