But how a mod can be banned in a discord server by random member? To do this they need admin status right? Or am I missing something.

No, they are not. There is two Discord server involved to run the attack, and the scammer does not have any admin status on both.
So, the scammer finds a victim(targeted admin/founder) in server A, and then they try to find if those victims have also joined some discord server that the scammer is also part of it. On server B who the victim and the scammers are both of a member, the scammer is accusing the victim of is a known scammer, as a result, the mod on server B is banning the victim. Which resulted why the victim believe the one who sends a DM is a real mod since he/she got banned.
I tried to play around with inspect element on the Discord web app, and now it shows:

I don't know when was Discord started to show that message, but if they just recently show it because of the type of scam on OP, at least there is an effort to inform their users about the danger of it.