I don't think it works that way for Infinite Dragons.
They can move about the timelines however they please. Nozdormu is still alive and will create Murozond eventually. Who will probably perceive how we killed the previous Murozond.
The only way to kill him for sure is to kill Nozdormu and stop him from ever existing. Which is a concept I think the Deaths of Chromie was hinting at. The only way to really kill an Infinite Dragon is to kill their bronze self.