Is the young man considered unjust for not fulfilling his promise of marriage after both parties repented from zina, especially since the girl desires a concealed marriage (sitr) with him?

You have no right over this young man, and he is not obligated to marry you. You are not wronged by his not marrying you; rather, you are the one who wronged yourself by complying with him in committing indecency. However, if he repents, it is more fitting for him to fulfill his promise and marry you, as long as you are repentant. We pray that Allah will conceal your faults and compensate you with good, as long as you have repented a true repentance.