Sango obviously does not have doubts when it comes to his capability to destroy the city, or Getha achieving her destruction on the city.
There was a reason that he made it a year interval, that was so he could get Getha ready before then, and disperse away whatever assumptions that it would not be possible for Getha to accomplish it.
Because he never doubted himself or Getha's ability to destroy the city in a year's time, he never bothered himself to think that there would be another who would think contrary to his own thinking.
Failure was not in the list of things he accounted for, so why would anyone else do that?
"Listen here monster. I don't know what hole you crawled out from, but destroying a city is an act that will only be possible in your imagination, not reality".
Zita managed to get herself to stop laughing so she could properly speak to Sango and Getha without having her speech in broken form.
