This is true for the user, but for very large organizations, since many @usernames are based on real names (2 out of 6 in this topic so far, and especially in corporate settings), it can be rather common for admins of large instances since a significant % of the population changes names at least once in their life, usually in the time frame where they use online tools. 
Atlassian decided to use usernames as primary keys in their products, and hundreds of admins have been complaining about it for 10+ years, because changing usernames has been impossible/broken that entire time. Granted, with Discourse it isn’t quite that bad because the usernames can actually be changed but it still leaves broken trails of damage around the app.