last interaction constraint example seems wrong

In https://github.com/dmlc/xgboost/issues/4075#issuecomment-458280942, a very precise definition of feature interaction constraints is given. In my view, the last example in https://xgboost.readthedocs.io/en/latest/tutorials/feature_interaction_constraint.html


is wrong (if the definition is correct).

Why? The ancestor nodes on the right-most branch have feature set {x_0, x_1}. So the last split should also be on either x_0 or x_1.

Am I wrong?