You are wrong. A zani is still a muslim. Ask your mufti.If someone who did zena/fornication/adultery is not muslim, how could s/he be judged by syaria law? by islamic law?
If someone who did zena is no longer muslim, why must they be stoned?
And all fornicators are not stoned to death, only married ones.
Unmarried fornicators are flogged in public.