Falseness can be determined in an antirealist ontology in two ways: hypocrisy, and ethical consequences. For example, if someone claims a belief in science, but thinks climate change is a hoax, that’s hypocrisy. We can point out that science says climate change is happening. And if someone claims to value kindness, but misgenders trans people, we can point at the dysphoria caused by the actions.

Now, these two tactics won’t work on people who don’t mind being hypocrites and don’t care about ethics. Namely, fascists. We deal with fascists using violence. This includes “soft” violence like denying them freedoms, and hard violence like physically or emotionally harming them. If everyone agrees not to be friends with nazis, then nazis are gonna be lonely and maybe they’ll stop being nazis in order to gain friends again. If that doesn’t work, we can throw soy milkshakes at them. If that doesn’t work, we can put them in a room where they can’t hurt anyone and make them see a therapist. And if that doesn’t work, we can shoot them with a tank.

If someone has the entirely self-consistent ideological view that putting Jewish people in gas chambers is an axiomatic good, then I am willing to commit acts of violence within the bounds of the Geneva conventions to enforce My subjective belief that putting Jewish people in gas chambers is bad. That action doesn’t require realism, it just requires that I be strongly committed to subjective principles.