You're correct, it's not enumeration, but it's easier than pure brute force. As the article itself suggests, if the company does use a predictable user name format (and most do), it's pretty trivial to look up their employees on LinkedIn, create a list of likely usernames, and then check if they exist.
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.
How does knowing the AWS username allow you to send a more effective phishing email? Especially if the username is just the user's... name. That's public info.
