The natural transformation α : F ⇒ G is not specified properly b/c when expressed in compositional form you also have to specify the subscript for the natural transformation & it is an equality instead of an isomorphism, i.e. if f : a → e then αₑ ∘ Ff = Gf ∘ αₐ. There are highter categories where what he has written down can make sense but in the context of the current exposition it is not correct as written.