Not exactly, I'll explain.
If you are not logged in, and you receive a full-page "no permissions" screen, on most skins there will now be 2 login forms: The navbar, and the main body of the page.
If you use the login form on the main body of the page, and your password manager does not already have an entry for your site, it will offer to save your username and password. It will correctly read the password you have filled in, but it will use the placeholder text (which is translatable via the vBulletin Phrase system) as the user name.
For English forums, this is "User Name". For multi-lingual forums, this would vary based on the chosen language of the user. This is why it's not feasible to exclude these entries from the Non-Existent Usernames list - the actual wording used would be different based on languages.
The way this would trigger the Watcher in vBSec is if the user does not know about this caveat with their password managers, the subsequently log out and try to login again, trying 5 times using their saved input, never once noticing the password manager is filling out "User Name" instead of their actual username before they click Submit.
In short, it's incredibly rare and requires the user to not pay any form of attention whatsoever.