In هُمْ بِمُؤْمِنِينَ, the last consonant of هُمْ is a mīm in the word itself—an original mīm. In مِنْ بَعْدِ, the initial letter of concern is written nūn sākinah, converted toward mīm before bāʾ by the earlier rule of qalb. This is a converted mīm.
Both can involve nasal sound before bāʾ, but the author’s condition makes their origin relevant. Inspect the written consonant and the rule, not only the auditory similarity.
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Original mīm versus converted mīm
In هُمْ بِمُؤْمِنِينَ, the last consonant of هُمْ is a mīm in the word itself—an original mīm. In مِنْ بَعْدِ, the initial letter of concern is written nūn sākinah, converted toward mīm before bāʾ by the earlier rule of qalb. This is a converted mīm.
Both can involve nasal sound before bāʾ, but the author’s condition makes their origin relevant. Inspect the written consonant and the rule, not only the auditory similarity.