مُسْلِمُو originally had wāw as its nominative marker. Adding the first-person possessive yāʾ brings a wāw and yāʾ together in the same word.
The book describes the underlying form as «مُسْلِمُوْيَ». Here we explain why that form changes, rather than memorizing an opaque spelling.
Key point: the original nominative marker matters even after its separate form disappears.