Why is لَنْ يَفْعَلُونَ incorrect as a governed plural form?

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

• Lan cannot be used with plurals
• The plural wāw must be deleted
• Subjunctive requires deletion of inflectional nūn
• The imperfect prefix yāʾ must be deleted