Intaṭaqa yantaṭiqu is Form VIII from n-ṭ-q (ن ط ق), used of girding or belting one's clothing at the waist. Lam negates and makes the following imperfect tantaṭiq jussive, reflected in the final sukūn.
In this passage, ʿan tafaḍḍul belongs to the picture of comfort: she is not compelled to tie up her clothing to undertake chores. The complete idiom matters more than forcing an unrelated abstract meaning of “virtue” on tafaḍḍul.
Freedom from household labor functions as a marker of privilege in the poetic milieu. The grammatical construction and the social values implied by the portrait are distinct layers that should be examined separately.