I get that, I’ve been “SoNick” or some variation of it for a few decades now. However, a friend of mine uses different - but thematically similar - usernames for different services, when I asked her about it she explained that as a girl on the internet she has attracted more than a few stalkers; changing up the username makes it harder for them to find her on different services.
The difference is that ActivityPub clients currently only display the username part and hide the host for some reason. This can lead to confusion, I don’t get why the full address isn’t displayed by default.