Most common folks dont care and arent as paranoid as us about this kind of thing so they usually use the same password(or variations of it), in most services whether those are their bank accounts, email, game username, forum username, (etc you name it) and the same applies for usernames(relating to your question). This makes the general population of our planet, that has access to internet easy targets for hackers, companeis and even their own family/friends, since once you discover one password or username you can try variations of it in their other services and chaces are that you will be able to find a match.
Now going back to your question of: Is “different usernames” as good as “different passwords ?
I believe so. In this age where privacy is becoming harder and harder to get as big corporations such as google and microsoft find better ways and improve their technology to use their advertsingbots to track all our activities and throw us Ads related to our activities, the best thing we can do is make it harder for them to track us down. I believe this also applies for security purposes, if you use the same username in several websites it makes it easier for a hacker or anyone to track you down and have a full picture of all your activities on internet, most people think this is not a bad thing but it is. Also Add to the fact that this kind of phyloshopy of putitng the same username also makes it easier for your friends, family, relativies colleagues discover all that you do in the internet by the simple fact that you werent careful enough to use different usernames. Which might lead to annoying/embarassing situations depending,since those people in your real life know where they can find you in the virtual world.
For example a hacker can discover your password in X service that you use. He might try to also check if your password is the same in the Y service, since you use the same username in both, even if the passwords are different he might have already collected enough information about your tastes, desires and your other password to try similar matches. In order to not have to go through this burden the best thing you can do is use different usenrames, passwords and if possible different emails or alias to make it harder for them.