There are lots of omegaverse stories that don’t involve marking, it depends on the author preferences.
Different worlds, different rules.
Gurl the reason the omegaverse exists is because a supernatural fic writer on ao3 wanted a man to get pregnant. It's dumb to put rules in fantasy genre, every omegaverse is different.
I think their marking here is more like they have similar pheromones scent can be in the chapter 45/46 (?). I read the novel where the omegaverse as marking with saliva during shared heat/rut. So there wouldn't be bite marks or obvious marking sign. But the saliva would seep into each other's blood after the bite.
This is an omegaverse but why is that not a single time in 100 chapters they never mentioned about marking??! Isn't that the very nature of omegaverse! The whole reason omegaverse exists yet they never thought about including this topic once?