It shouldn't be controversial to make this argument, but defensive reputations die hard and resurrect slowly in basketball. Once a player gains a title it's hard for them to shed that, regardless of what substantial improvements have been made. Arguably no player has made a bigger defensive leap than Kevin Durant this season, even surpassing a player responsible for the last two DPOY's.
In the past Durant was too weak to defend big men, being routinely rag dolled by the more physical players far above his weight class. Now he's proven capable of going toe to toe with likes of Karl-Anthony Towns and a rejuvenated Kevin Love in the post.
Kevin Durant Defense On Karl-Anthony Towns
He does a much better job of holding his ground and uses his measurements to make difficult shots even harder than usual. He can be the best help defender in a lineup absent of a traditional rim protector, has the mobility to switch out into any guard in this league and can hold his own in the post against bruisers.
This season has even shown him defending LeBron James well in isolation, a player who gave him a lot of issues in the 2012 Finals where he found himself often in foul trouble.
It's not an effective strategy for James to pulverize Durant in the post anymore
While Durant isn't as great as Leonard in staying attached to the hip of guards and nullifying screens, his ability to be a presence both on the perimeter and in the paint gives him more impact on his respective team's defense. In positions that Greg Popovich will scarcely subject Leonard too, the Warriors rely on Durant to succeed in on a game to game basis - averaging 1.7 blocks per game, allowing just 47.5% shooting at the rim, and playing a substantial portion of his minutes at power forward and even at times center.
His versatility is exactly why the Warriors have the number one defensive rating in the NBA, despite losing traditional defensive bigs in Andrew Bogut and Festus Ezeli.
"Kawhisolation" is flawed, and wouldn't be effective against Durant
It's a popular theory that teams are sacrificing their best opposing player to take Leonard out of games, but that is a less practical strategy against a top weakside defender at the position. You cannot eliminate Durant from defensive possessions due to his ability to cover ground so well. Kawhi Leonard likely isn't making this play against Anthony Davis.
There has not been a substantial decline in Leonard's opponent FGA (9.4 ---> 9.2) so there's little reason to believe teams are avoiding him much more than last season. The Spurs also will elect to have Danny Green defend the best opposing player for large portions of games, so it's not as if he's having to stick to only the best threat on the floor.
Durant has defended over 200 more shots than Leonard, and is allowing opponents to shoot 3.5% below their average compared to Kawhi allowing 1.5 below his average per NBA.com. The difference between them in attempts is not teams being reluctant to attack Leonard, but rather Durant carrying more responsibility for his respective defense.
It's easy to say that sharing the floor with Draymond is the cause for his increase in defensive metrics, but the Warriors will stagger their stars more than any team in this league. He has already played 540 minutes without Draymond on the floor allowing a 1.08 opponent PPP in that span, which would measure well among the best defenses in this league.
In the Warriors second most used lineup featuring Durant, he shares the floor with ground-bound David West. In 156 minutes that lineup has allowed a stout 1.03 defensive rating, largely because of how strong Durant has been as a help defender.
It is true that sharing so many minutes with Pau and Parker skew Kawhi's advanced metrics (and it's because of this that I will not use them against him), but the other side of the spectrum suggests he needs someone as the second line of defense in order to maximize his defensive potential. That used to be Tim Duncan, now it's Dewayne Dedmon. You don't ask him to be the security blanket for a defense in the same way the Warriors have relied on Durant.
None of this is necessarily taking away from Kawhi, but rather highlighting just how incredible Durant has been this year and how his improvement across the board has led to a Warriors defense ranking first in the association. It's time we look at him as more than just a scorer, but one of the best league wide on both ends of the floor.