> No, enumeration would be if you can get the entire list of usernames somehow.
It's still a form of enumeration.
Once you know the generation scheme, you can always enumerate some form, perfect or otherwise.
If there are rate limits, or if the search space is vast, this is notably more limited (even if arguably still enumeration)
Enumeration means a specific thing, that you get a full or paginated list of things and all you need to do is hit it. This isn't enumeration at all. It's like you saying you know the bucket keys to a hashtable so you can enumerate it by hitting them one by one, except in this case you don't even know the keys.