For a moment I was fool. The scammer who knocked me was using the username of the original admin, in his (scammer) bio profile.
This is already an old trick but new telegram users can still fall for this. Good thing you double checked. I don't understand why telegram is not removing that space for bio since it is being used by scammers to put usernames of other people.
That's really a stupid thing, for a moment, very much pro people can also get the impression that that's the username and he is the real person. If telegram doesn't allow to put @ at bio, that would certainly be okay.