The basic moral argument of anti-natalism is not that all life is necessarily unworthy and therefore it is always unjustified to create life. Therefore a life that is found worthy by those who live it does not contradict anti-natalism. Anti-natalism consists of much more than a single argument saying that life is always unworthy or that it is always better never to have been.
The possibility that a life will be found worthy by those who live it certainly exists, but the possibility of guaranteeing that whoever is created will live a life that is worthy in the eyes of those who live it does not exist, and taking a risk that whoever is created will feel that her/his life is not worthy is morally wrong.
The possibility that a life will be found worthy by those who live it certainly exists, but the possibility of getting consent from those who will live every life does not exist, and it is morally wrong to put someone in such a vulnerable situation with so many vulnerabilities that will inevitably happen, without that someone’s consent.
The possibility that a life will be found worthy by those who live it certainly exists, but the possibility that people will not harm others during their lives does not exist and of course harming others is morally wrong.
The possibility that life will be found worthy by those who live it certainly exists, but the possibility of creating someone for their benefit does not exist, and this is for the simple reason that if someone did not exist before we created them, how can creating them be an action that somehow benefits them? In order for an action to be good for someone, it has to move someone from some specific state to a better state, or to a state that was in the interest of that someone to move to, but non-existence is not a state that someone can be in, and therefore in creating a person there is no moving someone from any state to another state at all. Since there is no one before it was decided for someone to be created, there could also not be someone that it was in her/his interest to be created before it happened, and therefore the creation of a person is never for her/his benefit or in order to promote the interest of the person being created.
The possibility that a life will be found worthy by those who live it certainly exists, but the possibility of creating someone without imposing most if not all of the circumstances of the existence of that person, including a certain genetic load, certain parents, a certain immediate environment, certain formative initial experiences, a certain uterine environment, initial stimuli during the pregnancy, the era in which one was born, the society into which one was born, someone’s culture, origin, gender, sexual orientation, physical characteristics, mental characteristics, and many other factors that determine the created person’s life and personality, does not exist.
All these and many others are common anti-natalist arguments, and they are valid regardless of whether there are cases of created people whose lives are worthy in their eyes. Therefore this possibility does not contradict anti-natalism.
So it is not true that one case of a worthy life is enough to contradict anti-natalism. However, since creating a life that turns out to be unworthy is morally wrong because it harms the one who was created, but preventing life even if it would have been been worthy is not morally wrong because it does not harm the one who was not created, and since every case of reproduction can create someone who finds life unworthy, and it can never be known in advance, the possibility of one case of an unworthy life is enough to justify anti-natalism.