Seeing our Guru as Buddha - in that way we mention specifically taking refuge in Guru
You can say more specifically: “I take refuge in Guru, I take refuge in Yidam, I take refuge in Dakini.” And “Buddha” does not mean only “Buddha Shakyamuni”, but there are many Buddhas. Guru Rinpoche, for example, is also a Buddha.
By the external look He is different from Buddha Shakyamuni and we call him Guru. [But] we don’t [often] call Buddha Shakyamuni “Guru”. We call him “Buddha”. But as for Guru Rinpoche, we call him Guru.
So that is the way you call your own Guru, because the teachings request us to see our Guru as Buddha. So your Guru is different from Buddha Shakyamuni. So we still count our Guru as Buddha, so we say: “I take refuge in Guru.” It is correct because in that way we mention specifically taking refuge in Guru.
Hungkar Dorje Rinpoche, “Instructions For Dharma Practice”