Many don't know much about the Ominous Reaper before he began to terrorize mid-rank demons but they know that is has only been a couple of months since he became known. That means he has gone from being a low-rank demon to a high-rank demon in only a few scant months. The fact that he evolved so quickly from being a mid-rank demon also proved that he really killed those 300 high-rank demons. Where else is he going to get the needed resources to grow his mark of sin so quickly?
It doesn't matter if he killed them as a mid-rank or high-rank demon, the fact that he could kill them without those who revived having memories of their death is enough reason for high-rank demons to flock to him. That single feat proved him to be future demon lord material and they want to be a part of that. The news that he could do something that only the demon lord should be able to do only proved that conjecture.
