Why is هَادُونَ used rather than هَادِيُونَ in the taught sound masculine plural?

Ism Manqus: Case, Dual and Sound Plurals

• Final ya is deleted and the preceding kasrah changes to dammah.
• The noun changes into alif maqsurah before ونَ.
• The form becomes feminine before the plural ending.
• The original ya changes to hamzah.