The defense wasn't getting turned around in one season. That it's where it is in year 2 is pretty remarkable. Kalani may have been a defensive coach prior to being HC, but he's not close to the defensive mind and tactician that Hill.
Full credit to Kalani for the hire - which is what good leaders do. He got a guy who can better identify what the needs are, who will meet those needs, and how to go get them. And KS can go in to close the commitment, which is where he excels as a warm fuzzy that parents and players love.
But the reality is the dumpster fire defense under Tuiaki was as much at the hands of KS as the hands of E. Tuiaki wasn't ready to be DC, it seemed to be too big of a job for him, and KS didn't (or wouldn't or couldn't) have the ability to help him right the ship on that side of the ball.