In the story, M+M, how can Martin's "betrayal" of Mariela be seen as imaginary?

Martin's betrayal is complicated, for at the time he joined the communist party, he and Mariela had already ceased to be friends. In this way, Martin’s betrayal of Mariela is mostly imaginary, for he had no actual association with Mariela at the time of his joining. His conversation with Mariela, in which Mariela accuses him of being a traitor, is completely imaginary. In this way, the author implies that Martin himself thinks he is a traitor to Mariela, even though in reality, he has not actually broken her trust by joining the communist party.