Tatmīm (التتميم) adds a meaningful benefit that improves the expression even where the earlier wording would not itself cause a serious misunderstanding. Iḥtirās, by contrast, guards the sense against an unwanted inference.
Near the chapter's close, the source considers «وَيُؤْمِنُونَ بِهِ»: mentioning belief after an earlier description gives that aspect independent significance and honour.
A writer should not call any added adjective tatmīm automatically. The specific new value of the wording must be shown, or the distinction from padding disappears.