Qinw is the hanging cluster of date-palm fruit. Mutaʿathkil evokes its crowded offshoots or profuse fruiting structure. The focus is therefore not the cluster's color or outer skin but its many overlapping branches.
In ka-qinwi, ka- marks comparison and qinwi is genitive; al-nakhlah completes the construct. The following verse develops the locks and intertwined strands that make the cluster image particularly apt.
A simile of hair to silk would chiefly suggest softness; comparison with a date bunch chiefly suggests profusion and interwoven clusters. The same subject can receive different poetic meanings through different comparators.