Siri did not deny irregularities at the conclaves, and in fact alluded to them. He did not speak openly about it because he felt gravely bound by "the secret" (as he called it). From his perspective, he "stepped down". Objectively speaking, however, third parties might conclude that he stepped down under duress. So it's hard to say what he would have been thinking at any given point. So, if he never thought of himself as exercising papal authority, then he would not have appointed a successor, nor would he have been teaching in any other capacity than in that of a private theologian (and so therefore would not have been protected by the charism of infallibility).