I don’t understand your issue. It 100% could work that way as Microsoft could simply block Gmail requests because, I don’t know, let’s say they are constantly receiving malware from Gmail servers in attachments.
Email from Gmail to Outlook would fail but email from Gmail to Yahoo to Outlook would not as Yahoo to Outlook is not blocked.
This feature can be achieved very easily by them without making accounts. They already have a list of Instagram usenames. When a new user makes an account on Threads, check the username against a simple list of Instagram usernames and if it matches, throw an error stating the username is not available.
Why does that person need to have an actual account on Threads?
Samsung’s OneUI does this by default for all connections .