Thank you for reporting this. I think that page meant regular options. No custom GraphStyle. I usually resort to something like below. I hope people will realize a neat trick which will become obvious if you remove AspectRatio->1. This is a method to keep vertex size scaled relevant to 'Graphics` size and not stretched. Some may find also THIS useful.

style={
    DirectedEdges->True,
    GraphStyle->"Prototype",
    VertexLabels->Placed["Name",Center],
    GraphLayout->"RadialEmbedding",
    VertexSize->Scaled[.07],
    AspectRatio->1
};

TreeGraph[RandomInteger[#] <-> # + 1 & /@ Range[0, 25], style]