Most of these SSRI medications essentially elevate seratonin in the brain, which basically dampens / inhibits dopamine in certain brain regions. So, by reducing anxiety and inhibiting dopamine, it can improve speech fluency for some stutterers. They don’t work as strongly on dopamine inhibition as antipsychotic dopamine antagonists, but by mildly dampening dopamine they can still improve fluency for some. Reducing anticipatory anxiety also helps with fluency too.