Watch it again:
Jokic starts the shooting motion jumping with the right foot still inside the arc and it NEVER touches the ground outside of the arc. This is a two point shot.
A step back three looks like this:
In each one of these examples, you'll notice that the player establishes two feet behind the arc before jumping for the shot. If you begin the shooting motion INSIDE the arc, it's a two. This is why Durant's shot was counted as a two when his foot was on the line, because one foot technically leapt from inside the arc, which is exactly what Jokic's did.
When you're playing 1 on 1 or 21, we have to clear the ball with both feet. It's the same principle. Jokic jumps from his right foot and never brings it down OUTSIDE the arc-- it's still airborne as he jumps from his left foot.
The incredible awkwardness of the shot confused the refs (hell, the league) and they counted it as a three, when it's factually a two point shot