The imperative also takes nūn of emphasis. Heavy nūn strengthens the command; not all dual or feminine-plural slots permit the light nūn in the source chart.
CommandActivePassive
Plain, third masc singularلِيَقُلْلِيُقَلْ
Heavy nūn, third masc singularلِيَقُولَنَّلِيُقَالَنَّ
Light nūn, third masc singularلِيَقُولَنْلِيُقَالَنْ
Plain, second masc singularقُلْلِتُقَلْ
Heavy nūn, second masc singularقُولَنَّلِتُقَالَنَّ
The development قُلْ → قُولَنَّ is not just a mechanical appendage: the long vowel also returns in the emphatic form.