That aspect is like email. If I have the email address Limeade @ protonmail.com, someone else can sign up for Limeade @ gmail.com without it affecting me. Gmail doesn’t ask all the other email servers around the world if anyone is already using Limeade. If you wanted to send me a message, you can’t just send it to Limeade @ anything.com and expect it to reach me.

Depending on which service you are using, sometimes usernames show the full username @ domain.tld format by default, others just show “username” and you have to click to see the full info that here on Lemmy and the wider threadiverse I am not just Limeade, I am @Limeade@beehaw.org (the extra @ in front is the standard format for writing usernames on the fediverse).