You're absolutely right that restorative practice can't be facilitated effectively with someone who's not willing to admit to wrongdoing. That said, a lot of the incentive to not admit to wrongdoing comes from feeling fear of incarceration, fear of social isolation, and fear of other consequences that someone may feel may come from admitting that they caused harm. This is backed by research.
If people who have caused harm are approached by a program option whereby they're not going to be incarcerated as a consequence and they're actually going to have some measure of community support in being accountable for the harm they caused, the research shows that the rate of people consenting to a restorative practice is quite high. They usually want to, when that's an option.