Q. If Moses wrote the whole Torah, how could he have said about himself, “There has not risen since in Israel a prophet like Moses” (Deut. 34:10)?
A. Some say that the final verses in the Torah were added by Joshua after Moses’ death. Others say they were written by Moses himself, at God’s command, with tears in his eyes (Talmud Bava Batra 14b-15a).
The notion that a person could compose his or her own epitaph is not unknown even today. An Australian curiosity is the gravestone, now in the Pioneer Memorial Garden at Botany cemetery, of Joseph Marcus, unofficial spiritual leader of the Jews of New South Wales in the 1820s.
Probably no-one else could write Hebrew so he may have prepared before his death in 1828 the text of the last two lines of Adon Olam which others must have transcribed onto his tombstone.