Question: Could You please explain why in this technique one does not repeat a mantra?

Babaji:

It is not necessary. The mind needs to learn to become quiet on its own. The use of a mantra is a preliminary exercise. The ultimate truth of the mind is God or The Self, so when the mind settles the consciousness goes back to there. If you wish to recite mantras, then recite them first, and after that go to meditation.

It is the aim of all sadhanas for the mind to become quiet. If the mind becomes quiet that is the real worship. When the mind is quiet then it automatically goes introverted towards God.