How does Melchor characterize Munra's discovery of Norma's true age?

Munra is quite adamant about the fact that when Luismi shared with him that Norma was only 13 years old, he blew up at his stepson, telling him that he must be out of his mind. Munra was quite forceful, according to his own account, so it appears doubtful that Munra knew the extent of the age difference between 18-year-old Luismi and 13-year-old Norma.