Vertex-transitivity of the automorphism group of a digraph

Question

I am trying to understand the theorem 3 of Cycles in graphs and groups by Kantor.

Theorem $3$ If $G$ is a vertex-transitive group of automorphisms of a digraph $\Gamma$ with outdegree $d \ge 1$, then $G$ has exactly $d$ orbits on $X \left(\Gamma, G\right)$.

So, $G$ is the automorphism group of $\Gamma$ so it should always be vertex-transitive by definition. Why do we need to mention it explicitly?