I'm pretty sure they meant that using a password manager means you don't have to remember the _username_ for every site.
I've started to just generate a 10-char alphanumerical string with the password generator feature and use that as the username on new accounts, starting to update them retroactively, too (by scrapping the old account and making a new one).
Thats great, then you can use the same for your password.
Really, why is that?
because if you get your account compromised on one site/forum because of lax security on their end, you don't want all your accounts compromised.
How will your accounts be compromised if they are all different? A key is no good if you don't know which lock it leads to.
Because it might be tied back to your email account or some other common denominator. Using different usernames is good for dox attacks like this but doesn't necessarily help you if your email is compromised. So you need different passwords (AND MFA!!!!)