Why may يَفْعَلْنَ after lam be in a jussive position despite keeping its nūn?

Negating the Imperfect with Lan and Lam: Subjunctive, Jussive and Full Paradigms

• It is indeclinable with nūn al-niswah; its visible shape need not change
• Nūn al-niswah is inflectional nūn
• Lam cannot govern feminine plural
• This is an ordinary perfect tense form