The knock on D’Antoni is his teams don’t play any defense.
While his Suns teams were never considered defensive stoppers, they weren’t as bad as you think, plus their point differential was always one of the best in the league.
Nate McMillan while a big name, his defensive prowess has been overstated according to Pro Basketball Talk , which leads me to believe that this is more to appease fans than actually help the Lakers defense.
Fans are naive and believe anything you tell them, so if they feed Lakers fans the line McMillan is defensive specialist it might ease their concerns over D’Antoni’s hiring.