You are allowed to be moving. You don't have to be set.
"Block-Charge: An on-ball, block-charge situation occurs when contact is made between an offensive player (who is moving in a particular direction or trying to change directions) and defensive player. The defender is permitted to establish his legal guarding position in the path of the dribbler regardless of his speed and distance. To get into a legal position, the defender needs to establish himself in the path of the offensive player before contact is made, thus “beating him to the spot,” and before he starts his upward shooting motion."
Durant's upward shooting motion (i.e. when he gathered the ball and began raising it from his waist before he planted his right foot to go up for the layup) began far outside the restricted area and before LeBron had planted either of his feet.
Durant is a huge player with enormous strides so he can plausibly begin his shooting motion on a drive far away from the basket. The rule is not whether the defensive player was established in his spot before the offensive player reaches that spot, it's whether the defensive player established before the offensive player started entering his scoring motion (as opposed to dribbling). This makes sense because the offensive player should have an opportunity to change directions or pass the ball. Otherwise the defensive player is just jumping in the way of a player knowing that player will run into them.
There's no way LeBron was in position, he would have had to get there much earlier.
It was a block.