because nami did not even land the last blow it was zeus.
zeus is meant to be nami weapon but it was not nami using zeus that defeated ulti. it was zeus alone while nami was watching in shock:
nami had no merit in this she missed the attack and then zeus did all the work after she gave up.
imagine if zoro failed to defeat king but then enma starting moving by itself and cut king head while zoro looked like suprised pikachu. it would not be zoro defeating king in that scenario.