Niels defended the 3, 4, and 5 at various times, all capably. He rebounded effectively. He was a very flexible defensive player. On offense, he was a spot-up shooter, but that inflexibility was not a problem when you had Bazz and Boat in the backcourt. I don't see that Niels was a rigid player, certainly not rigid like Vance Jackson who couldn't defend anything and demanded to play the 3 and get shots.
I think KO just means that he adjusted his offense and defense to the capabilities of the players he had, e.g. slowing the game down to compensate for lack of rebounding by Amida and Deandre or lack of footspeed by Niels. There's no one system, in other words, there is a flexible repertoire of approaches and the best approach for his group of players is what KO will choose. He is rebelling against having a single "system" that everyone has to fit into.