False. Usernames can be the same across different domains, but they need to be unique within the same domain. For example, two people can have the same username on different email platforms (e.g., john_smith and john_smith), but within a specific email platform, usernames need to be unique (e.g., john_smith cannot be used by two different individuals simultaneously on the Gmail platform).
So following your logic and common sense true.
Yes, following the logic and principles of most systems, it is true that usernames are typically unique within a specific domain or platform.
