Possible inaccuracy 

The article says:
"another way to think of it is that the order that the leaves would be visited in a depth first search must be the same after the operation as before"

Correct me if I'm wrong, but I believe the order of visiting the leaves in a DFS after the rotation will change, but the in-order traversal will remain the same.

DFS traversal before right rotation: QPABC

DFS traversal after right rotation: PAQBC

In-order traversal before right rotation:APBQC

In-order traversal after right rotation:APBQC