“Aslama al-kāfir” refers to someone who was not Muslim and embraces Islam during a Ramadan day. From that point the person becomes responsible for Islamic worship, but earlier days are not assigned as retrospective missed Ramadan fasts.
The original Arabic term is preserved as text; the explanatory wording uses “new Muslim.” The practical focus is worship after conversion rather than inventing a debt for pre-conversion Ramadan days.