“Wa-khtāra Mūsā qawmahu sabʿīna rajulan”
“Moses chose seventy men from his people.” (al-Aʿrāf 7:155, excerpt)
The phrase indicates selecting a subset of the people, not choosing the entire nation as seventy persons. The relation of selection is explained through an understood “from among his people” (min qawmih).
Arabic verbs may govern complements directly or through prepositions according to usage. Here the intended subset relation is evident from the number and action.
The point of the reconstruction is to make the selection intelligible, not to insert a new recited particle into the verse.
“Taʾtūnanā ʿani al-yamīn”
“You came to us from the right.” (al-Ṣāffāt 37:28, excerpt)
One interpretive expansion considers pressure ‘from the left’ as well; that is a gloss, not additional recited wording.