So most likely everyone knows that If you try to signup with non alphanumeric characters in your username, It will give you an error saying Your username can only contain alphanumeric characters (letters A-Z, numbers 0-9). I tried this with the character - and It said that, but if you change your username through the CLI, It will let you use the character - in your username (This only works with the character-), So please fix this bug and remove from the signup page that you cannot use the character -.
The usage of hyphens in usernames seems to be intentional, as it is clearly stated that hyphens are allowed.
However, I believe this is an old feature, as changing the username via the CLUI does not have any new restrictions or limitations that the signup page have.