Aside from the recent unroll.me hate, I think that’s just a mistake in their writing.
Mistake? AFAIK there isn’t a way to retrieve your password. https://unroll.me/a/login
If they only use gmail/google oauth, they wouldn’t have a password. They’re just implying that the login uses oauth which uses your google account email/password.
I used to have an unroll.me account, and don’t have a password recorded for them.
I guess if they truly use OAuth for everything, then it’s fine.
For Gmail, Outlook, and Yahoo they use OAuth to access your email. For AOL and iCloud they just need your password for IMAP access.