I'm thinking it was made up. No doubt Nishi wouldn't surrender (to have done so would have been considered very dishonorable in his culture), but I think the part about U.S. Marines appealing to his ego is, well, probably something his family conjured up.
Pride was a big thing to them.
Sounds made up to me. Only a few intelligence and commanding officers would have known the bio of the Japanese commander, and they wouldn't have had much reason to capture him.